Don, these have been discussed in several different threads on quite a few acoustics oriented sites, but I don't have any quick links saved - basically, for overall room control the more surface area the better things work - corner placement gives the most even absorption, because all modes terminate in corners. The simplest and best thing to do is entirely fill a triangular area in each corner of the room with absorbent, such as rockwool or rigid fiberglass, somewhere in the range of 2.5 to 4 PCF -
BTW, you need to update your profile information to include a location; this site is subscribed to worldwide, and making recommendations or giving measurements is easier when I know where you're from; otherwise, you may get metric info when you're used to imperial, or vice versa.
Second best approach would be the same locations, but just stand some 4" or so absorption panels across the corners, floor to ceiling - hermetic seals are NOT needed here, you're just placing the absorber in the path of all sound waves that reach the corners, and giving them a variable depth of air space behind the absorbent (hence "broadband" absorption) - this approach won't give quite as much absorption in lower frequency ranges, probably about the same amount in mids/highs, and saves some $$ in materials if you're on a "snudget
(new word I just coined - contraction of "snug" and "budget" - so sue me
Some commercial manufacturers make the tubes with hard and soft hemi-cylinders - one half of the cylinder harder (more diffusion) and half softer (more absorption) but I've not tried these so won't comment other than they don't LOOK to me like they'd work very well. One company stands a whole bunch of these around an area and claims they make things super peachy; again, no experience yet so no comment, only doubts - although, if you had ENOUGH of these around they'd have to do something (besides just emptying your wallet, that is)
Best bang for buck in nearly ANY room - option # 2, above. That, and treating first reflection points with spaced absorbent, spaced clouds overhead, and (for CR's) deep traps at the rear, and you've got a good start - (This is all based on SMALLER rooms - larger ones can benefit from less bass trapping and more diffusion, as long as you can get at least 3 wavelengths (at the lowest diffusion frequency for your units) away from the diffusors... Steve