I think the 501s are a step up from the 402 although the 402 is a wonderful amp. I think the primary consideration here is the price difference. I was going to get the 402 but found the 501s on demo for less than the 402 and could not be happier with the amps. Like Entdoc, I think there is a significant difference in soundstage. I really recommend the 501s and think they are the kind of amps you would hold onto for a long time, even when you decide to change speakers. As for the C45, I'm not a bog fan and think the 501s run a lot better with the C2300. That said, I might look at a non-Mac preamp altogether.
Best Amp for McIntosh sys.- MC252, MC402, or MC501
I am upgrading a system that I have for about 40 yrs. Recently purchased a MCD301 sacd/cd player, MR85 tuner, and Paradigm Studio 100 series IV speakers. Still have C26 pre-amp and MC2505 amp (both recently serviced by MAC). I am happy with the sound but am afraid the days of shipping the amp back and forth for service are limited. The spks are rated "Suitable amp power range 15-350 watts" and "Amplifiers that exceed-power range are recommended". I don't intend to make another amp upgrade and am leaning toward the 501.