Here is an incredible pedal by Boss. Usually when I see/hear a modeling pedal it's close but no cigar.

Is this pedal as good as a cranked up 59 bassman? ...of course not. But what you could do is run it parallel to the amp. For instance y-cord or go through a mixer where half your signal hits the amp, the other goes through the pedal and then hits the amp.

Live this might be kind of a hassle so you run it straight. Boss is getting close with this though. I could see/hear where this could make a so-so amp stand up and bark.

Check out this video:

Nice playing by Johnny DeMarco also.

Here is a collection of reviews:
Most agree that this pedal really helps clean amps to be more versatile.
Leave the amp eq flat and use the pedal eq.
Tube amps love this pedal, doesn't do much for the small practice amps.
Great for Blues and Overdriven country.
Makes the bridge pickup of a strat have more bite.