I had the Mac 501's with my Maggie 3.6R's for about 2 weeks and the combo sounded muddy, slow, and the top end was very etched and harsh.
That said, those excellent amps found a quick home at a friends house with his big B & W's and sound absolutely stellar. Strong, huge bass, crystal clear highs.
Differt horses for different courses.
I do love Mac preamps with Maggies. I use Cary because it's a bit warmer sounding, and I like that with the Maggies.