About digital potentiometers: if you want to go expensive ($3.5/piece), you can get 2 channel 25kOhm 1024-step AD5235 with bandwidth of 125 kHz, THD of 0.009%, noise of 20 nV/sqrt(Hz) and crosstalk of -110 dB. That's just one example pot, there are others just as good.
These numbers mean that it's better than many opamps. This is worse than O2 performance, but still extremely good if this is the part that limits the amplifier.
The tricky part is that you'll need a microcontroller to run the I2C from buttons. Might as well toss in a DAC.
Edited by AstralStorm - 9/16/12 at 8:35pm