It is NOT impedance but a combination of low impedance and a high phase angle which defines a bad load.
Even a mediocre amp will drive a resistor. Some even at 3 ohms or MAYBE even lower.
However, throw in a highly reactive load and there you go.
http://stereophile.com/standloudspeakers/243/index4.html
http://stereophile.com/floorloudspeakers/1208thi/index4.html
http://stereophile.com/floorloudspeakers/1105thiel/index4.html
http://stereophile.com/floorloudspeakers/663/index4.html
None of the above links are to the Thiel models asked about, but certainly indicate a design philosophy which requires a first rate amp which can maintain into large phase angle loads.
Good hunting.
Even a mediocre amp will drive a resistor. Some even at 3 ohms or MAYBE even lower.
However, throw in a highly reactive load and there you go.
http://stereophile.com/standloudspeakers/243/index4.html
http://stereophile.com/floorloudspeakers/1208thi/index4.html
http://stereophile.com/floorloudspeakers/1105thiel/index4.html
http://stereophile.com/floorloudspeakers/663/index4.html
None of the above links are to the Thiel models asked about, but certainly indicate a design philosophy which requires a first rate amp which can maintain into large phase angle loads.
Good hunting.