devinplombier-
The CS5 and 5i are identical in their requirements. The only difference is the 5i has woofers that Jim improved via incorporating his motor shunts and focused gaps, which do not change power requirements.
The Apogee line, as I recall, had nicely resistive (non-reactive) impedance profiles above 6 ohms. No problems. But their efficiency / sensitivity was extremely low at around 80dB/W/M. So they needed lots of watts to drive them.
Thiel speakers share the nicely resistive, non-reactive impedance profile, but have very low impedance across the board. The CS5 used a unique method of low-frequency extension where the 2 sub-woofers were paralleled to halve the impedance in the very deep bass. The magnitude drops smoothly from about 6 ohms at 10kHz to 4 ohms at 1kHz to 2ohms at 20Hz (and about 1.7ohms at 10Hz. That punishingly low deep bass impedance occurs where there is little musical content. Predicted performance was better than real world performance because many / most amps become unstable/erratic when delivering large current flows demanded by low impedance, even if those flows are not large. Amp designs vary greatly in this area and likewise does the nature of the sonic malaise.
Krell and Pass address the issues very well, as do others that the marketplace has identified over the years. You all have your finger on that pulse far better than I do.
I have personal experience with the early 100 watt Classe DR9 designed by Dave Reich with robust current capability. When bridged it supplies 400 watts at 8 ohms, 800 at 4 ohms and 1100 watts at 2 ohms. I had mine hot-rodded by Bill Thalmann at Music Technology for significantly improved definition and noise performance over stock. Perhaps I’ll take them to Duramax’s place to compare their performance to his later, more sophisticated units.
I wish that Jim had paid more attention to impedance magnitude. The stew could have been stirred to raise the values by a couple of ohms without significant sacrifices. But as it is, extreme care in choosing the amps driving Thiel speakers pays high dividends.