Even with the 20.1 if you upgrade the crossover, it may indeed make a big difference - not having anything to do with the crossover, but because of the implications it has on the amplifier relationship. You can go with something like Spectron Musician III for the bass amp, which is 800 watts/channel, and then go crazy with nice sweet sounding lower powered amp on top end. You get 4X the power out of an amp when using an active crossover, in part because of the lack of need to control the other sides of the audio spectrum. For less money you can go Bel Canto kilowatt amp.
Imagine an 'effective' 3200 watts on that bass panel, and whatever amp you are running being dedicated to the higher frequencies.
Definitely go XVR1 or consider DEQX if you are so inclined. That is a whole world. Not sure what level of DAC you are at - while the DAC in the DECX may not be up to 10k DAC standards, the whole package you get may have a net improvement on the system.
For the 20.1 I wouldn't go under XVR1 or DEQX, although the Bryston unit will of course also work just fine. Nothing less than that, though.