Worth upgrading my sub?

I'm currently running a pair of Revel F32s with a Denon receiver as my preamp, and the signal is crossed over to my subwoofer at 80 Hz. I'm in the market for a "real" preamp, but I'd like to continue crossing over the low frequencies to my sub, which I'll do using a Marchand XM44 crossover. I'm doing this because my room is such that the mains themselves have a terrible response curve between 20 and 80 Hz (over 25 db difference from peak to trough), while the sub has a very smooth response in this frequency range. I'm also limited in the amount of bass trapping / wall panels I can use; since I live in a studio apartment, any acoustic treatments need to be very livable, as I literally live in my listening room.

Currently, I'm using an SVS PC20-39 sub, which is 10 years old. This sub reaches down low, but I'm wondering if a more modern sub would deliver "cleaner" or "tighter" bass than this one. Not that I'm unhappy with the sound; I'd just like to know what I'd get from upgrading the sub. I also realize that dual subs might be better than a single sub, though I'm not sure I'm looking to use a dual sub setup.
I would consider upgrading the sub cable. I have a Rel B3 and the stock cable is horrible at best. I personally have a Nordost baseline made to Rel specs. Huge difference for the better. Something to consider. I'm not saying to get Nordost but try a better cable.
Based on these measurements: http://www.avtalk.co.uk/showthread.php?t=10959 distortion doesn't appear to be issue, but group delay might be. The sub is amazingly clean to 25 Hz.

I saw your preamp post... I assume you vetoed the idea of using a pre/pro as a preamp. Something like an Integra DTC-9.8 would handle the bass management without an external controller and allow you to run balanced lines to your A21 amp. Plus, you'd get room correction. Just my opinion, but a "real" preamp is a step backwards.
I'm pretty set on getting a tube preamp; I'd also like to use my own DAC, which I believe would not allow the use of room correction features.