Hi Dr. Jam,
On the "economical" side of things, getting another piece of MC402 is not a bad thing. This is something I had pondered after having upgrading from the MC501's, albeit a little controversial in perspective:
1. This would save a "trade in" loss (from MC402 to MC501s, no matter how small a sum it may be).
2. 2 sets of Mc402's when mono'ed would yield some 800 watts per channel. Now, thats a cool 300 watts upper from a pair of MC501's. Now, thats 3/4 way from a pair of MC1201's and its no mean feat! It will blow any pair of speakers in the market to musicality!
3. So far, no one had ever made a direct comparison between mono's MC402's vs MC501's, so for sure, its not like the MC501's would win hands down at this stage.
4. Should the day comes when you decide to go vertical bi-amping instead of mono amping, you have 2 great stereo McIntosh amplifiers to do the job.
Controversial thinking, maybe ... well, its all up to you. Happy hunting and make the decision that makes most sense ... ($$$$ and musical performance) ... and of course, its still a McIntosh ...
Cheers!