On my quest for a better 12" box I spent some time talking to dealers about the RCF offerings. The HD12A look good as does its bigger brother the HD32a. (in Australia) the HD12A is close to the price of a PRX612, no price on the HD32a yet but it will obviously be more expensive.
The spec's on the compression driver in the HD32a are 2" with a 3" voice coil. It crosses over at 800hz The 3 in HD32a stands for the 3" voice coil the 2 stands for the 12" woofer (those crazy Italians) and its got a 600 watt amp. These would be well within my budget and probably something I can order in to try.
From talking to a few places today it sounds like the ART 422 would be a match (maybe even better) for the PRX612 - so I imagine the HD32a with more power and a larger compression driver (than the PRX612) should be a step up.