I ordered the Nitsch without a DAC and use it, just like the RebelAmp, with external DACs, so I have pretty good comparison options.
First of all, it must be mentioned that the Rebel has significantly more power and is therefore able to drive significantly more energy-hungry headphones with ease.
In terms of sound, the Rebel is a slightly more neutral amp in direct comparison, which harmonizes extremely well with a very large number of headphones and is actually always a good all-round recommendation.
The Nitsch, on the other hand, is a revelation for me as an old, die-hard Sennheiser fanboy.
In my long time dealing with headphones, I have never found an amplifier that drives the Sennheiser 600 series as cleanly, controlled and tonally correctly as the Nitsch.
If you compare the amp/headphone pairing as a jockey with his horse, which I like to do, then the Nitsch as a jockey has his horse, the Senni, perfectly under control to overcome every obstacle.
But that doesn’t surprise me, as Nitsch advertises a “tuned” HD600 on its website.
I personally use the HD650 directly on the Nitsch and the HD600 on an xDuoo TA26, which the Nitsch is connected to as a preamp.
As a DAC for these combos, I use the Ifi Neo iDSD, whose amplifier section I am surprisingly rather disappointed with, but whose DAC is fantastic.
All in all, the Nitsch is not quite as versatile as the Rebel, as it has a more colored sound than the Rebel, which does not harmonize as perfectly with all headphones as it does with the Sennheiser.
Incidentally, in this price range you should also include the AUNE S17pro, an amplifier that is in my opinion the best all-rounder in this price segment and is absolutely rightly rated so highly in the reviews.