A bad surge protector can limit current but a good one will do nothing more than allow the same current from the wall to flow with minimal additional voltage drop. RFI filtering may help if you are in a very high RFI environment and your components have very poor RFI rejection (unlikely these days). There is no way that I can imagine that a surge protector would add bass nor affect the imaging.
It is possible the power filter is reducing high-frequency noise and thus making the bass sound more prominent. But I think it far more likely that this is placebo or cognitive bias. Our perception of the sound can change randomly based upon many factors, including mood, other noise sources in the area (fans, kids, pets), and so forth. Minor changes in the position of speakers, listener, room furnishings/decorations, and such can have a large impact. I once moved a picture out of the first reflection spot and the sound got "duller" but "clearer" and appeared to have more bass due to the lack of high-frequency reflections. Once we (think we) hear a difference it is almost impossible to "unhear" without running a blind test.
Is it possible you made some other, seemingly minor, change that affected the bass or overall sound?
The best way would be to measure and confirm, but again if something else was moved it obviates the test.
The wall connection goes through a power supply designed to isolate the audio output from the wall outlet. The wall connection is rectified and filtered so any minor changes in wall power have essentially no impact on the audio output, and the internal circuits themselves typically have additional (and very high) rejection of power supply noise, making it very difficult for something like a surge protector to do anything to change the sound except in a negative way (by reducing the voltage to the component).
FWIWFM, HTH, etc. - Don
