I found the 402 a very solid contender for many many speakers.... I've heard the 802N, but not the Diamonds. hearing the 501s on the same speakers as the 402 & 602, were the costs all the same I'd opt for the 602 and not look back.
I thought the 501s sounded more critical by far than the other two amps. All of the audition was fueled by the top tier Mc transport, and mc SS preamp. So the electronics were pretty fair to middlin'.
naturally as important as is the squeaker + amp combo, I feel any Mc amp listed here will work electronically.... the caveat I'd submit now is this:
There's more to the quality of the resultant sound than just those two factors.
you gonna run the exact same source, cabling, and so forth as your friend?
it is something to think about, so I thought to mention it.
otherwise... your results may not be those your heard elsewhere even though the same drivers and power were in play..
BUT....IF the Ds are better integrated from the mids to the top, and the Ds top end is easier than the Ns, i could see where the 501s would attract AND even with longer listening sessions be satisfying.
if however the Ds tweeter is on par with the Ns, I'd opt for the 02s... which one is up to budget it would seem though.
I'll wave ba bye with this.. bought rightly them Macs usually hold their value. Buying a preowned but very nice 402 to try out ought not to provide much of a deficit if it simply does not float your boat and with some ... a little libation and maybe a steak, you ought to be able to coax your 501 friend to swing by with his 501s for a bit of comparison listening.
By golly you ought to know that way for sure!! then either keep or flip.