They have more bite and match up well with a Les Paul. Some Rivera models offer Marshall and Fender channels. Better yet, both channels are designed to do clean and dirty sounds, though typically you'll use the Fender for clean and the Marshall for dirty. The solid-state Roland Blues Cubes are also worth looking into. Third, look for features that would make it useful on stage and in the studio.
Construction plays an important role as well.
Finally, the amp should have character, yet be transparent enough to allow the character of the ax to show through. This is because if you decide to go the rout of the modern-style guitar amp, you're still settling on two or three sounds. The best way to keep the amp sounding fresh is to have a couple of guitars to play through it. If you are the type to change your mind every six months, however, your pocketbook would fare better by using pedals. Next time, we'll take a look at the guitar rack system.