Need help - HP amp with "proper" balance control

2-chanel "level matching" an xlr input pretty much requires a dual-knob system.

you MAY be able to get alps to make you a 4-chanel balance pot, but you may also rather spend the money on soemthing else.

i vote towards a double pot arangement, in the interst of saving a decent bit of cash if nothing else. you will probably get stuck with the 2-knob arangement, as the "split knob" pots stax uses are somewhat hard to find.

Unfortunately there's very little in the way of audiophile-grade equipment with balance controls. My Melos SHA-Gold has a decent one that supposedly doesn't degrade the sound, but IIRC later production runs didn't have it (Melos went out of business a number of years ago). Fits your requirements for sound, has a ton of inputs switchable from the front. No XLR in but has an XLR out along with 2 RCA outs (one active, one passive). You'll have to find one used.

Typically, if the balance control is implemented as a pot, it will degrade the sound to some degree, which is why most audiophile gear doesn't have them - despite the significant overall imbalances many systems have (not to mention the cases where your ears are naturally imbalanced). Your best bet would be to get someone to build you a standalone balance control box to go between source and amp. Then you could use almost any amp you like. DACt makes a CT4 discrete balance control that balances 2 channels from -3db to +3db in 0.25db increments. For all practical purposes, it shouldn't degrade sound quality (like any good stepped attenuator). If you're not a DIY'er you'd need someone to wire it up and put it in a box for you. I'm going to try to get one built for myself, as many headphones have imbalances of up to 1db or more.

Edit: Unfortunately the CT4 is not balanced - and couldn't be wired for true balanced XLR inputs/outputs. It could be wired for unbalanced XLR ins/outs - I don't know if this meets your requirements. I also don't know if there are any truly balanced balance controls

stax "amps" with balanced inputs use a truly balanced 4-"layer" alps "blue" 50k audio pot. both left and right halves are tied together, and you may adjust the balance between.
the problem is that the pot is made for them.

funny, i was about to say Melos SHA-Gold too. My balance control is photo-smooth with a remote to boot. No XLR inputs, but it does have XLR outputs - but not balanced outs.

Have you considered getting a preamp to do the balance control? That way you could use any headphone amp you want.

I recall Mapletree Audio having a a balance control unit for their EAR+ kit. It might be worth looking into.