Hmm...I'm not much for 'warm', because that turns into 'thick' VERY quickly for me, but I do prefer a little warmth to absolute tonal neutrality. I suggest a used conrad-johnson Premier Eleven-A. It's rated at 70 Watts per channel into 2, 4, 8, or 16 Ohms. Again, if the Mac 275 had enough power, the Eleven, too, will. Elevens are available for c. $1500 here. I just sold my 2nd or you could buy mine for $1450.
If the Eleven doesn't have quite enough power, you could buy a pair of Twelves ('the same' amp but single channel for 140 Watts into 2, 4, 8, or 16) or merely buy another Eleven and run both in mono. The Twelves show up here occasionally; the Elevens often.
If you have more money than I to spend on this stuff, buy a new Premier 140, which is a current-generation 140WPC stereo amp that EVERYONE loves.
WOW there's one right here--http://cls.audiogon.com/cgi-bin/cls.pl?ampstube&1113954455 . If $4700 is a little much, and BOY do I wish I had it, there's also a pair of Twelves here now--http://cls.audiogon.com/cgi-bin/cls.pl?ampstube&1114028794 , for a few-hundred$ less. Personally, I'd buy the Twelves because I LOVE monoamps, but if you have no unnatural attraction to single-channel amps, the 140 is the better buy, IMO of course.
.
