Perhaps the output impedance of the headphone out mess up with the Abyss as you suggest, but I thought it was due to a more appropriated current / voltage balance from the Denon taps,
The output impedance doesn't affect the frequency response of planars due to their flat impedance curve.
But it does cost power of course, the system is a voltage divider.
The Abyss has 44ohm impedance, so if the amp puts out say 20Vrms and the output impedance of the amp is <0.1ohms (like on the speaker terminals), basically all the voltage drops over the load of the headphone drivers resulting in a high current (20Vrms would be way too much for the headphones).
If the amp has say 300ohm output impedance, only 14.7% of the 20V drop over the headphone drivers, severely limiting the effective power output of the amp.
as I have read that planars like current.
Well, current "I" flows in relation to the voltage "U" applied to the resistance "R" (or impedance "Z" in this case, but doing AC calculations with complex numbers is a huge hustle, so just use ohms law as an approximation) of the load. Power then is calculated as either I*U or U²/R or R*I².
So the headphone has an impedance (resistance) and a sensitivity, the latter meaning how much sound pressure (SPL) the driver produces either for a given power (the Abyss would be 88dBSPL per 1mW of power) or voltage (ignore that for now).
What I assume people mean when saying "planars like current" is that they usually tend to have a low impedance and a low sensitivity, therefor drawing more current than many electrodynamic headphones.
There is some truth to that as a general trend, it's certainly true for the Abyss, but saying it like that is technically inaccurate.
I must listen louder than that
You don't. You don't listen to a constant 0dBFS signal. The average level of music is a lot lower. Still the amp must be able to supply the peak power required without clipping.
110dBSP is more realistic.
110dBSPL is a good guide value for the level of power the amp should be able to put out to provide enough headroom and not clip with normal use in my experience.
Of course a more powerful amp is always a possibility, you can just not use it after all.
The speaker terminals of your AVR are a very good choice for driving the headphone, it can put out a massive 30.2Vrms (114W @8ohms according to Amirs review) before clipping. More than enough to destroy your headphones twice over. There are some adapters that make it a safe thing to do, otherwise just don't turn the volume up too high I guess
As far as your original question:
I'd say get an amp capable of 10Vrms and 1.5W of power, a little less would be fine too. Wich one - have a look at the reviews. Just don't buy something that performs worse than you 3700X in terms of signal quality.
