GT-10B is the "best" overall multieffects for bass. The M9/M13 stuff has better sounding drive effects, reverbs and delays IMO, but the rest of the effects are subpar compared to what you get with the GT-10B, modulation in particular. Also, the GT-10B offers amp sims and a ton more hardware options like XLR DI outs, expression, amp control, FX loop, and on and on.
Once upon a time I had the M9 in my GT-10B effects loop. However, now I'm happy to just use the effects in the GT-10B and pair them with judicious EQ and not need to haul some other large box around with me.
Once upon a time I had the M9 in my GT-10B effects loop. However, now I'm happy to just use the effects in the GT-10B and pair them with judicious EQ and not need to haul some other large box around with me.