so this tactic worked before (the wet/dry pcb, that I still haven't gotten around to ordering

I think you could design a very nice headphone monitoring system for use with high end cans. You already have the Dual class A, smack a couple of those together for balanced outputs, implement an EQ so one can alter the response, add crossfeed functionality, a nice switch for volume control, maybe some stereo/midside processing functionality (stereo width adj?) etc...
I like the headphone amp I use with my HD800's (the one in my Dangerous Music Monitor ST), but they are awfully powerful with my 800's, resulting in having to use the lower range of the volume potmeter, where L/R tracking is pretty terrible. The adjustment range becomes very small, easy to hurt your ears and easy to use a range that skews the panorama.
And I would like one for the bedroom. But I need that eq adjustment (based on response files of the 800 that I like, that I now do digitally with a generic parametric eq), the 800's are just so thin and treble'ey without it. And since I got a nice set of expensive balanced cable throwed in along with the cans when I got them (that the seller seemed intoxicated about), why not go balanced. Even tough from my knowledge, I don't really understand why this is a thing with headphones and their short cable runs in generally interference free environments... but double is always better, right?

some inspiration for units I have in mind :
https://spl.audio/en/spl-produkt/phonitor-3/
https://rme-audio.de/adi-2-4-pro-se.html (well sans the digital part I guess. Or, if you by any chance are an amazing digital designer as well, do go ahead

In my head such a processor fits right into your product line, that's generally directed towards mastering needs. Maybe just a processing board to be added to the ClassA project?
Here's hoping....
-magnus