the old transformers were good and fine.
done right, switch mode power supply is just as good, if not better. long term reliability remains a questionmark for the behringers, but so far, so good.
lab gruppen, crown, qsc, powersoft, at al have mostly abandoned the 'big iron' amps in favor of the greater efficiency afforded by the newer technology. these are the brands that power rock concerts and imax theaters. behringer's model in many cases is to copy other's designs, save on the r&d and support, and sell at a budget price to take market share. as a result, you can get some pretty good stuff, cheap. not the best, but pretty good value, particularly at the lower price points.
there are a ton of bass options. a pair of ported 18's can be done pretty cheap (stereo integrity 18ht or Dayton 460ho), inuke3000/6000dsp amp depending more on what is best match for the driver impedances, in big low tuned ported cabs is a proven model.
one possibility Large Ported Dayton HO18's
can be pretty much made into any shape you want with the same performance so long as cab volume remains about the same. lots of shallow builds that are large. jbrown15 has some cabs that you might like. another guy is working on something quite similar. not sure if he would mind me sharing his concept.
i'd suggest that you upgrade your mains to something with controlled directivity and high sensitivity. the speakers that you have may not be the best for bringing home the imax experience.
read up on room modes as that will influence the bass response quite a bit and in ways that aren't always obvious.
search around a bit and read through some of the build threads that are kind of like what you want to do and folks will be happy to help.
if you really want to do it right of course, a small investment in some measuring gear will go a long way (even an spl meter can be used with test tones to get some idea of what is going on).
here is jbrown15's build:
LTD-M18 Sub Build
plasmad pocket sub is another oddball subwoofer designed with very shallow depth. definitely a creative solution.
The plasmad pocket sub