Been there, done that. And so have many others. The opinions are mixed.
I really didn't like the results. Excessive "coloration", loss of mids I really like, etc. It sounded cool at first until I found myself constantly tweaking my setup to be heard, and I was missing the precious upper mids that it cuts out.
The SansAmp doesn't make amps sound "better", IMO, and GK is no exception. Those heads sound excellent on their own. That's partly why my rock/metal band arsenal is fronted by a 2001RB.
IMHO, the true glory of the SansAmp is in direct recording and ampless gigging. Perhaps if there were a way to turn off cabinet emulation on the SansAmp, it'd sound better to me through my bass rig. But if I wanted that, I'd look to the EBS MicroBass II.