The only real purpose of the mdf box was so that I did not have to drill holes on the underside actual Urie speakers bottoms. So essentially, the mdf and speakers are vibrating together as one.
That's fine! Some people do it that way. It also has the advantage of adding mass to the speaker cabinet itself, which is a good thing.
I understand that speakers are designed a certain way (weights, dimensions, materials, etc..) to achieve a certain sound, and that adding the mdf may effect that design.
999 times out of a thousand (invented statistic, but you get the point...), that will IMPROVE the sound. Speaker manufacturers go to great pains to make their cabinets as rigid and massive as they can, but there are practical limits... such as shipping costs, for example: A speaker with a 6" thick concrete case around it would be great, but the shipping might be a little steep. Probably not too many buyers, either: Nobody wants to have to hire Chunk Norris AND Arnold Schwarzenegger, just to lift the speaker out of the box, and onto the desk! (Which then collapses under the weight...
) Silly analogy!
Anyway, just wanted to relieve your concerns that adding mass and rigidity to the box might change things in a bad way: it wont. It will either be neutral, or good. I'm betting on "good".
I am hoping that I didn't mess something up with this design.
It's fine! It will work. But I still would suggest considering pads around that, to keep it all happy...
However, the blocks are already there, so that part can't really change - and the mdf board is already cut to into pieces ready to become boxes sop that would be sort of a waste of money since I can.t really use it elsewhere.
The blocks are fine! No problem. Might even be over-kill, but it's a hell of a solid, massive base! There's no need to do anything to that.
My concern here is that, with a large powerful speaker like that, there's quite a bit of mass slamming around inside the box, which in turn can potentially cause the box to want to move forwards and backwards on top of the pads. Those Sorbothane studs you have are designed to support the weight of the speaker, acting vertically downwards, and to deal with any vibration in that direction. That's what they are tuned for, and that's what they will do. But if there are lateral forces acting side-to-side, or front-back, those pads are not going to be able to deal with that correctly, because the speaker is so tall. At low volumes, it's not going to be an issue, but when you turn it up to eleven, there's a lot of inertia going on inside, with the woofer mechanism chugging away merrily: action/reaction. There mass moving at high speed, the suddenly reversing direction and going back the other way, then repeating the cycle, ad infinitum. However, since the bottom of the cabinet is restrained by the studs, but the top is free to move, there's a potential for that inertial mass thumping away, to cause the entire speaker to rock back and forth on top of the pads.
This might not actually be a real issue, in your case! So don't sweat it too much. Do some testing first, and see if you even have a problem. If you do, then we can figure out solutions. If not, then you can sleep soundly again. So set up one speaker completely, turn it up to eleven, play lots of bass-heavy music, and see if the speaker actually does "rock" at some frequencies. Also run some pure sine test tones through the speaker at high power, and ramp up the frequency slowly, so see if there's any type of strange resonance that might cause it to "rock" at one specific frequency.
I don't want to scare you and say that you definitely will have this issue, and it will make your sound terrible, and tear your room apart! I don't like scare-mongering... There's just a potential for this to happen. Your speakers are so massive already that the effect might be negligible. It can be an issue with smaller, light speakers run at high power, but yours might be heavy enough to not even be bothered.
I would assume that with Johns method of mounting a speaker so rigidly that the actual speaker box doesn't move at all, would also have a similar effect as adding weight?
Yes... sort of! But with a speaker that large, it would be an "interesting" exercise...
For the lateral movement of the speakers, those pads I am using are not just flat pads - they are called stud mounts, and they have a threaded piece sticking out each side maybe 1/2" or so, something like this:
I'm familiar with those, yes. I have used that exact thing for HVAC compressor isolation mounts, and other things. And I've actually seen one case where the pads worked fine for up-and-down motion of the compressor, isolating perfectly during normal operation, but they were totally shredded and torn to bits, dumping the compressor down onto the mounting brackets, when they failed during start-up, due to the lateral forces acting on them as the compressor spun up to speed... At very low speeds, the compressor speed ramped up through the resonant frequency of the pads, and at that specific frequency there was enough vibration in all directions to rip the Sorbothane to pieces. Not the fault of the rubber, of course! The fault of the design. You should take other precautions if the isolation mounts will ever experience vibration at their natural resonant frequency, due to the amplification of the forces.
Now, don't worry! That's NOT going to happen with your speakers! They are not HVAC compressors, and they'll never be producing frequencies in the resonance range, that could provide such destructive vibration. But it does illustrate the point that there are OTHER forces acting on those studs, not just gravity and vertical vibration.
This should prevent any lateral movement cause by normal operation of the speaker (vibration).
Mmmmm.... maybe not! See above.
However you are definitely correct that these could be tipped over if you just press on the side.
Red flag!
once the walls are built around the speakers, there will be no access for anyone to be able to push the speaker as it will be inside the wall.
Maybe people can't get in there, but other forces could... have you ever seen the massive air pressure changes that a large speaker cone can produce inside a box?
I'm aiming at getting somewhere less than 1/4" between the speaker and the soffit face.
That's side-to-side, but what about front-back motion? There's nothing to stop the speaker moving in that direction... and it's quite possible that a real person COULD push on the front face of the speaker, forcing it backwards into the soffit. Either by accident, or on purpose....
I always try to get that gap down to 5mm, then use 6mm high flexibility closed-cell foam all around the edge, to get the air-tight seal. You don't want air huffing and puffing though that gap, as the woofer woofs...
You had actually told me to private message you about this proprietary method you are talking about all those years back but you never responded!
That's weird! I really don't recall seeing that at all! Sorry about that.... Try again, if you feel up to it...
- Stuart -
Definitely! 
all good. I tested it WITHOUT the crossover as well so it was getting full range. I went real slow with the sine wave generator in all the low freq's to see if I could get a reaction out of them but nothing. Tried some deep house music as well and some other stuff with a deep kick and bass. No visible movement at all. the only time I can get visible movement (rocking, wobbling) is if i push it by hand. Also, although the insulation is loosely filled, there is definitely noticeably less rocking/wobbling that happens when I push the box now that is filled. I think just the weight of the insulation itself sitting on top of the box, and the friction along the sides, is somewhat dampening the rocking even further. I hope this is a good thing. There's definitely no flanking or overly compressed insulation anywhere. The extreme weight of these things gives it a very firm/solid feel overall.