Actually, this KM-X1000 has a power rating of 190WPC at 6ohms, which appears to be the minimum recommended load.
The power supply is somewhat light for this rating (2 x 10,000uf capacitors), so I would be careful on driving low impedance speakers. It may do okay driving 4 ohms, but keep in mind that many speakers can drop down to 2-3 ohms in certain frequencies (like bass frequencies).
I don't know that I would agree that THX amps can drive any speakers. At some point, the amp is going to reach saturation in the power supply, especially if you start wiring many speakers in parallel (dropping the load to under 1 ohm). This can cause heat and clipping (which is bad).