This is the cheapest option and sounds great:
I've seen some modeling users buying those Seismic cabs and direct mounting these amps in them as well.
The amplifiers used in these boards are very common to what is used in powered monitors and many powered speaker driver components. For the larger amps, you would have to add a power supply (referenced with each board type).
I've seen GSP1101's rack mounted into Seismic Audio 2x12 (for stereo) cabs using Fane Soveriegn full range speakers and having these power amps mounted internally. They sound bad ass! It's like having a Zentera, Vetta, or Millennium style modeling amp, but you have any choice of modeler to use.