Inner Sound in Portland Oregon
http://www.inner-sound.combut it won't be affordable
unless you are a star, or you have a very high-end gear.
If there is a visible fault on the circuit board, or connectors,
it might be cheaper, only if the flaw is indeed the problem,
but still cheaper to buy another FB-01, use it till it drops,
and keep your lame one for parts.
Without a visible flaw, the diagnostic cost is even higher.
I think the initial estimate is just under $100, only to tell you that it
will be xxx hundred to replace the main componant, to test further.
Maybe you could try setting up a multi-band EQ rack, to compensate for
the flawed output?