Ideally, a pre and 4 monobloc power amps... Or a pre and 2 stereo power amps, or an integrated with pre-outs and a matching stereo power, or finally, a pair of matching integrateds as long as they have pre-outs and power-ins. What you want to get to the stage of is having 4 identical amps, each driving it's own drive unit. You're still using the speaker's internal crossover so they have to be the same. With posher speakers you can get external crossovers, but when you get to that stage you're better looking at something like a set of active PMCs with built in Bryston amps and crossovers before them on-board. It all gets into proper silly money when you get to that stage.
The best I ever made the RTL2's sound was on the back of a single Quad 606 down a single length of 79 strand to each. The simplest configuration of the lot, just with a very, very good amp on the front. The brace of Delta's all bi-amped couldn't touch it. The RTLs just don't respond as well to bi-amping as well as something like, say my old Kef References that were a 3 way design and you could power the internal bass unit on one amp and the mid/treble on the other.
Personally, I'd run it with something like
this. Mine ran well with one of these in the past. Cracking little amp.
