Need more information:
1) Exactly what routing are you using ?
2) If the presets you mention are in DSP A, what is in DSP B ?
3) Does this happen with all presets ? If not, give the numbers of some that are OK.
4) Does this happen with the THRU preset ?
5) Have you tried swapping amps/cables ?
6) Presumably your bass is mono. Is your preamp mono out or stereo out ?