It probably needs regular maintenance. Also, what kind of speakers are you using it with?
It <IS> a bass amp every bit as much as it's a guitar amp. Note, two channels? Funny enough the Normal sounds better for bass usually & the Bass for guitar, but whatever.
What matters most is, has it been taken care of? Maintained? And, if you plug it into a "bass" speaker, of course it will sound mushy and weak. So the answers to these questions would determine any advice from me moving forward...