To be clear, I have 212s, not a 215.
All I can tell you is that the A51 gives plenty of power to 212s.
Also, nobody in this forum has really corroborated my preference of a high bias class A amp over a class D amp on JTRs.
In fact, there have been a number of 215 owners who felt that a pro high power amp was really beneficial on their 215s.
All that said, I liked the A51 with the 212s quite a bit more than the NC400s, even after room EQ.
I also liked the 212s MUCH more after fine tuning with Dirac. In fact, Dirac made a bigger improvement than the amp.
Anyway, to describe the difference more...
JTRs put out flat treble.
If the treble is harsh, it can be irritating if you don't roll it off.
If the treble is smooth, the treble isn't irritating...just clear and live sounding.
That was the A51 vs the NC400 to me, even after room correction to the same target curve, which normally made all amps sound the same.
A class D with a higher switching frequency than the NC400s might be able to sound identical to the class ABs on JTRs.
If I were to start from scratch with 215s, I think I would try Emotiva XPA-1s & an XMC-1. Emotiva has their summer sale going on (on top of those club discounts). The XPA-1 has lots of power and is high bias class AB. It may not sound exactly like the A51s, but I bet it would after some Dirac magic. Also, no fans!
The XPRs are switching amps, so following these theories, XPAs would be preferable.