What are your goals? An upgrade could mean almost anything.
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Main issue: The amplifier MUST be of common-ground design. This eliminates all "bridged" or "balanced" amplifiers; and many but not all monoblocks--but the monoblocks that are capable of common-grounding will need a jumper wire connecting the negative speaker terminals.
My Polk SDA 1Bs are well-driven by an Aragon 8008BB; but since the Aragon is not a truly common-ground amplifier, I also need a short wire connecting the negative speaker terminals of the two channels. The 1Bs are nearly unlistenable with the Aragon's measured 20 ohms of resistance in the ground circuit between the two channels--a $5 jumper wire clears this up instantly. Adcom 5xxx amps are similar, as are some Parasound--and certainly others as well. It would be worth your time to put an ohmmeter onto your amplifier negative terminals and see if your current amp (or any amp you're considering buying) is TRULY common-ground.
The best money I ever spent on audio--aside from that $5 jumper wire--was to upgrade the crossover and generally refurbish my SDA 1Bs. Three or four hundred dollars in capacitors and resistors, a pair of inductors, a fresh pair of tweeters (two were still good) and some detail work--and the speakers are better than new.