I have Sonar X2 and want to get a good 6 channel mixer/control service that allows me to record all 6 channels on my computer.  

Current Beringer 4 channel mixer uses USB. I can get Channel 1 - in as Left, Channel 2 - in as Right - or single stereo mix of both. I have no way to separate out channels 3 and 4.  

Any particular reason you want a mixer as opposed to just a 6 input audio interface?

(posted at night, but the post is gone, so short version...)
Surface+Interface+Mixer solutions are no longer on the market. May be the only product left in that direction is Zoom R, but from all reviews it is more "ok" then "good" in all categories.
 
There are Interface+Mixer without surface (so the mixer should be pre-configured with Computer to work in standalone). F.e. many Motu interfaces.
 
Most digital mixers support multi-channel recording and some can be used as surfaces. But big, expensive and with relatively hi latency.
 
Possible solution: an audio interface which works as pre-amp in standalone (f.e. Tascam US 16x08) with outputs connected to simple analog mixer and separate Control Surface (f.e. Faderport for one channe, X-touch for multi).
Such combination can work standalone and can record, without rewiring and with supported in Sonar surface.