The short answer is yes, plugging a preamp into the effects return of your amp will bypass the SWR preamp and allow you to have a seperate EQ for your upright. If you plug your electric bass into the SWR's regular input, then you can use the Effects Blend knob to select between the two basses. This works well, but if you run a line to the PA there are some other things you must consider as I have recently discovered. Check out this recent thread
here where I pose some similar questions regarding my SWR WM12 and a doubling gig.
Many electric bass amps (inlcuding SWR) have low impedance inputs and preamps that are not voiced with upright bass in mind. Using a preamp as a buffer will usually improve the tone and bypassing the SWR preamp by plugging the preamp into the effects return may help even more, although you may loose some headroom depending on the preamp you choose. The pickup you use is another factor. My Realist seems to tolerate most electric bass amps well, but my BassMax sounds
much better through a preamp plugged into the effects return of either my SWR or GK. In fact, I never use the BassMax any other way.
Good luck, and let me know if you find something that works well.
- Steve