What is the worst that could happen? Is there really a significant difference between say the ART SGX2000 Express and the ART Nightbass? what would the implications be if I ran a bass through a guitar processor or vice versa?
There wouldn't be any damage done. Bass and guitar units use different EQ points on the controls so it may be harder to dial in a good tone. The effects in both of those ART units are pretty much the same.