It looks good anyway. You may just need to stick a solid divider in to shrink the internal chamber so you may get a little extra pressure behind the cone, those 6's may not need but a small fraction of a cubic foot chamber size. Possibly 80-100 cubic inches or so. .
Just some thoughts...Be Well
Alexander Tyrannicidae

You are correct, 60nDown, when the first 90 degrees of signal is being read by the coil, there is a vacuum behind the cone. Therefore the larger the rear chamber the less vacuum, and the smaller the chamber the greater the vacuum. It would be similar to having an air break for the cone, helping slow the foward excurson momentum..adding resistance for distance traversed by cone. With extra vacuum it may assist in reducing the over extention of the cone, and may help prevent the tapping/clapping sound you heard on the lower frequencies.
It also works in the opposite direction once the cone has returned back to starting position (back 90 degrees), in the third 90 degree part of the phase as the cone moves from center in the reverse direction the pressure builds and works in the same way. The w21 is like really cool sealed box where its basic nomenclature is somewhat like having a dual 4th order horn. You may also adjust the spacing between the front of driver and the sidewall, if the internal changes do not help as much as you desired.

Just some thoughts,
Be Well,
Alexander Tyrannicidae
The Mini WO is cool

I'm a firm believer in experementation. If you want to dry a smaller rear chamber volume, pop something solid in the rear chambers, about the same size, seal 'er up, and see how it does. If it sounds better, then you can figure out how much volume your solid objects took up, then take that ammount out of the rear chamber. If it sounds like poop, then out it comes.

Fine work 60. Perhaps drivers with a greater xmax along with that ability to handle more power are in order.

Thanks for sharing the pic.

before you scrap this thing, try DD 6.5's (digital designs) youll love them www.ddaudio.com   and their reasonably priced

hm the dds are 100wrms?

http://www.ddaudio.com/dd/caraudio/woofers.asp?series=W6.5

the peerless drivers in there now are 150? but i bet the dds are way better and take more but im not sure ill bother? its a GREAT h.t. sub right now, sometimes when im watching stuff im seriously distracted by the solid accurate bass lines eminateing from behind the tv   if i ever find a 350 wrms 6.5 driver i may re-consider ???

im on a diet

seriously.................................