I use an XP20 and X250.5 with my Tympani IVa's.
The power supply in Pass Labs amps are designed to provide abundant current into 4 ohms and have the heat sinks to sustain it.
500 watts into 4 ohms. I have found that to be sufficient.
The room is 17' W X 24' L X 13' to 19' H and the back wall is very open.