My SDAs have 400 w/p/c @4 ohms available via Aragon 8008, prior to that, Aragon 4004. My Polks are modified; they were rated at 4 ohms when new, but they're less than that now.
Stable to one ohm (maybe less) and "about" $700--$1300 on the used market depending on exact model and condition.
You may need a short jumper wire between the negative terminals of the two channels to bypass a pair of internal resistors that prevent the amp from being true common-ground. I'm sure there's an expensive way to do it, but my jumper cost < $5. A few inches of speaker wire would work, I used 12-gauge and some Rat Shack banana terminations. There are many amplifiers like mine, that are "almost" common-ground, except that they tie each of the negative terminals to chassis ground through a resistor. The SDAs need to have those resistors bypassed with a jumper wire. I've seen evidence that some Adcom and Parasound amps are built that way; no doubt plenty of others are, too.
Remember that Bridged or Balanced amps cannot be made common-ground with the exception of some Crown "grounded-bridge" designs. Far as I know, the "grounded-bridge" is patented or trademarked by Crown.