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 ?