I am staying with all McIntosh ...what speakers do you feel are a good match with the MC501s ....any suggestions ?You should treat them as you would a good tube amp, as the output transformers are the limiting factor to how nastier load they can handle.
I would say look for speakers that don’t dip below 4ohms and have less than -30 phase angle with that 4ohm frequency.
As in the Martins case above at 65hz a combination of 5ohms with -45 degree phase angle could represent an EPDR load to the amp of around 2ohms. And 65hz is the power area needed for bass.
EPDR= equivalent peak dissipation resistance
Wilson Alexia’s have an EPDR at 65hz of .9ohm!!!!!!!! Notice the simiarlity with the Danjo graph at 65hz?
http://www.stereophile.com/images/1213Walexfig01.jpg
Cheers George