RCA Bass Remote - about 20 bucks ish and can be found at www.sonicelectronix.com
If you go that route, let me know how it works out. I am going to try using my fader first, but if that doesnt work the way I want, I plan on using the remote knobs.
I believe that Wetsounds WS420 has basically used 2 separate bass knobs as to independently control tower and cockpit speakers. This is the conclusion I have come to studying their product. I dont see the need to spend the money on their rather simple design.