Seems I the thread I posted in is dead, so I'll repost here. I'd argue the NCx500 is close but not quite as good as NC400 and 1ET400:
I was wondering if it was just 2x improved compared to NC500 or compared to NC400, as the NC400 is already a whole lot better.
Looking at the NC400 - data sheet towards the end of the performance graphs, there are on the bottom left IMD 18.5 + 19.5 kHz, 100 + 100 W in 2 R (funny, it's a mono module - does that mean two modules on one power supply?), on the right there are no watts and ohms, let's assume it's 100 W in 4R. The two fundamentals are at -3 dB. First difference tone (~ HD2) at -100 dB, second at -124 dB. Third is in noise (-138 dB), fourth and fifth at -131 dB.
NCx500 18 + 19 kHz at -3 dB each, 100 W in 4 R: first at -114 dB (quite significantly better), second at -121 (slightly worse), third -131 (significantly worse), fourth -132 (same), fifth -128 (slightly worse).
If the half-bridge technology of the NC500 has been retained (NC400 is full bridge), that's quite an achievement to get to the level of the NC400, which is about even with the Purifi! Difference of NCx500 to NC400 is that higher orders are slightly stronger.
THD+N vs power in 4 R show similar differences. The NC400 reaches its minimum of 0.0003% at just over 50 W and is virtually the same above that for 1 and 6 kHz. The NCx500 has less noise and reaches its minimum of 0.00018% at 5 W, but it deteriorates significantly above that at 6 kHz. At 100 W and 6 kHz the NC400 still has 0.0004%, the NCx500 0.00055%. So less noise, but also less loop gain at higher frequencies.
Both are excellent modules! From a purist's point of view, I'd still say the NC400 is a tad better.
Edit: Purifi Eigentakt for comparison
18 + 19kHz 100 W in 4 R: fundamentals at -3 dB, first diff at -122, second at -128, third at -130, fourth in noise (-137). THD+N minimum of 0.00014% at 250 W.
The Eigentakt is a lot quieter than NC400 and NCx500 at full power. Of course, one would have to look at what buffer was used. Regarding IMD products, one can argue whether NC400 or Eigentakt is preferable. Again, the NC400 shines at high orders, while allowing somewhat poorer performance for low orders.