The desk etc. was straight off sketchup and I imported it as a bit of an indicator and didn't really place it in the room properly! Was the first/closest desk I found. It had everything on it. A bit lazy on my part (still learning to use sketchup)!
Ahh! That wold explain it...
My actual position is at the 38% position (thats where i took the initial REW measurements), and can adjust from there.
I'm sure you already know this, but the 38% "rule" is not carved in stone: it a good guideline, but you are allowed to move around a bit from there...
Just on the speaker on desk thing: is the "nastiness" reduced by a smaller speaker if it was sitting on say, an isolated mount?
There's many issues with having speakers on a desk or meter bridge. One of them is, as you guessed, decoupling: If a speaker is sitting directly on the desk, then it can potentially transmit vibrations into the desk, which makes it possible to have an issue with "early-early" sound: sound that arrives at your ears BEFORE hte direct sound from the speaker (because sound travels much faster through solid materials, such as wood and metal, than it does through air, so those vibrations in the desk could be re-transmitted into the air at the front of the desk, arriving at your ears before direct sound). But even if you don't get early-early sound, you still get vibrations, which you will hear, and do mess up the clean direct sound. Isolation pads can help with that, if they are chosen correctly (beware of the "one size fits all" vendors...). However, that's not the biggest issue: reflections from the desk surface is the biggest issue. With the speakers in that location, there will be strong reflections form the desk surface getting to your ears, an that's no good at all!. In addition, for the same reason, there will be comb-filtering going on: as you move your head up and down, of forwards and backwards, you will hear changes on the relative frequency response, due to the comb-filtering effect. Now, some people say that comb filtering doesn't matter that much, since your brain can get used to one pattern and then sort of ignore it... and that's true! So, as long as you can clamp your head in a vice while mixing, such that you never more your ears to a different part of the comb-filtering pattern, you should be fine!

But if you are like normal people, that move around all the time, then maybe that's not such a good thing... Then there's another effect: With speakers on the desk, the overall frequency response will end up with a dip in the mid range at some point, that depends on many factors. You can compensate for that dip, probably, with judicious use of EQ... provided that you are able to identify correctly, and tailor a filter to deal with it. And finally, there the issue of what I call "roughness": As the sound travels over your desk surface, it will encounter numerous obstructions, small and large, that deflect, diffract, reflect, diffuse, absorb, and generally modify the overall sound on it's way to your ears, in multiple different ways at different frequencies. This shows up as small but important deviations in the frequency, phase and intensity of the sound. In simple terms: your frequency response in the mid range looks like the mountains of the moon! Ups and downs all over the place. You can't equalize that fully, and it's generally a mistake to even try, unless you really understand what you are doing.
Getting your speakers off the desk and into stands eliminates some of these issues, and reduces others to various extents, so the final result is a great improvement. "All of the above" constitute the reason why many mastering engineers don't have a desk in front of them at all, or have a very low profile "minimalist" desk: they want to hear the cleanest possible sound, so they get everything they possibly can out of the path from speaker to ear.
Will definitely do that. So the absorber would go directly behind the speaker with the speaker centred to the absorber?
Right!
How much of a gap between the face of the panel and the rear of the speaker? Or would it be hard up against the panel?
No gap at all! The rear corner of the speaker should be right up against the panel, touching it. You want the speaker as close as you can get it to the front wall, for this reason:
SBIR-wall-bounce-distance-vs-frequency-GOOD!.png
SBIR, or "Speaker-Boundary Interference Response" is an issue caused by low frequency sound heading out backwards behind the speaker, hitting the front wall, then returning. It causes a very large dip in the frequency response, due to phase cancellation, and that dip depends on the distance between the speaker and the wall surface. For distances of a few feet, the dip is way way down low, in the region of drums, bass, keyboard low end, etc. You won't hear the frequency where the dip occurs, or will hear it greatly reduced: Sometimes known as "bass suck-out" among other names. You can't successfully treat such low frequencies with absorption on the front wall, because the frequency is too low and the effect is so powerful. So either you have to move the speaker so far away from the wall that the frequency is below the bottom of the hearing range, and that would be a distance of about FIFTEEN feet. Not possible in small rooms. Or you move the speaker up as close as you can get it to the front wall, which moves the dip up higher in the spectrum, hopefully to a point where you can treat it, at least partially, with absorption. If you can get it with a few inches of the wall, then the frequency will be up into the mid range, and treatable. For example, if your speaker cabinet is 11 inches deep, and you get a 4" gap behind it, that places the speaker cone 15" from the wall, so the SBIR dip will be at 220 Hz, which is treatable, but also less noticeable: A dip in the mids isn't as glaringly obvious as a dip in the low end.
Here's a real case:
SBIR-at-40inches-85hz--and-15inches-220Hz-GOOD!.jpg
With the blue line, the speaker face was 40" form the front wall, and you can see the large dip at 85 Hz. For the yellow line, the speaker was moved to be 4" from the wall at the rear, with absorption, placing the speaker face at about 15". You can see that the dip is far less serious, and moved up to about 220 Hz. That's the PRIMARY SBIR dip: you can also see the secondary SBIR dip at 2x85=170 Hz has completely disappeared. And also the third dip, at 3x85=255 Hz... gone! (There were other things going on between those two tests: it's not just SBIR! Don't get confused by that. Just focus on the actual SBIR issue, and how it went away by moving the speaker and installing absorption).
I had thought about it but with currently only running 5" monitors and not knowing exactly when i'll have the budget to upgrade i figured it would be a guessing game building soffits for speakers i don't have yet (if that makes sense). It's something i would definitely revisit when i upgrade speakers though!
Then design your soffits so you can replace the speakers easily!

I've done that a few times: made the soffits "modular", where you can simply slide out the actual module that holds the speakers, and is larger than the biggest speaker you would ever need. For example, at
Studio Three Productions, they started out with Genelec speakers originally, and that's what I designed the room for originally, but they thought the would upgrade at some point, so I designed the soffits "modular". About a year later, one of the Genelecs went flaky, so they decided to upgrade, and we selected Eve Audio SC-407's, which are MUCH larger, and I wanted to mount them vertically, not horizontally. But it was easy to do: they built new soffit modules, mounted the SC-407's in them, then simply switched out the modules with minimum down-time for the studio, and carried on working right away. Just a few hours. We then changed the room treatment and re-tuned it for the new speakers, to get optimum acoustics again, as studio time permitted, but the actual swap was done in a very short time. You could do something similar...
Would building soffits later negate any side/rear wall treatment I install now?
Not really. You'll need the side wall, rear wall, and ceiling treatment anyway, regardless of having flush-mounted speakers or not. You might need to modify it at bit, to deal with the changed acoustic response of the room, but that's not a big deal. What the soffit WILL do for you, is make your small speakers sound larger, eliminate a whole bunch of ugly artifacts (including SBIR from the front wall....), and is arguably the single most important thing you can do to your room to improve acoustics.
Moved some things around...
Getting better! But make your desk smaller... as small as you possibly can. Desks have an effect on the room acoustics: the smaller and more "open" it is, then less it will affect the room response...
- Stuart -