I play bass guitar and I was thinking of getting the BOSS Fender Bassman pedal for my bass rig. I know alot of guitars use the actual amp its self. But I've seen a bassist plug into that in amp form. I was wondering how would it sound on bass or is it a waste of money?
What is your bass rig currently?
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Don't bother with it. The Bassman pedal is designed to give a guitar the sound of a vintage Fender Tweed amp. That sparkle and singing overdrive when cranked isn't going to make your bass sound very good.

The Bassman was a fine amp and yes, it was created for electric bass. But it really was a better guitar amp than a bass amp; particularly in its original form.
"Maybe this world is another planet's hell?" - Aldous Huxley
I have a 61 bassman and I'm with fatal gear on this. Phenomenal guitar amp but not really anything special a bass player would want. I'm sure the pedal is just to give guitar players some vintage dirt. You might want to look into a tube preamp pedal or even a tube head if your still curious though.
What is your bass rig currently?
I got a squire Mike Dirnt p-bass and a stagg 5 string with a line 6 lowdown 175 and I occasionaly use the BOSS metal core. I saw a bassist with a great tone using a bassman and I didn't know if I could help achive that tone with the pedal
Nope. You'd need the amp head and the cabinet, plus whatever else he had in his signal path.
"Maybe this world is another planet's hell?" - Aldous Huxley