While that's true, one can consider separate functional blocks within a larger PCB to be 'separate boxes', and so the ground and power supply isolation that one gets from a balanced signal path also helps these circuit subsystems and functional blocks to work properly despite the fact that they share a PCB ground and power supply system. The typical idea now is that 'balanced' is useful only between separate boxes, and primarily serves to eliminate noise from interface cabling. The cost is that usually 2x the circuitry is required to provide a balanced signal path, but the benefit is that because signal currents are able to be kept out of ground and the power supplies, PCB layout is greatly simplified. An advantage of a balanced signal path is that, to a first order, a balanced signal path helps to keep signal currents out of ground, and given a few other techniques, allows signal induced power supply currents to be kept out of the power supples and ground. I'll put in a vote for using a balanced signal path. Oh, and XLR pin 1 goes straight to chassis.
That said, if you do need a variable gain control to accomodate more sensitive cans, after the PGA would be a good spot. Given that headphones wired up balanced tend to be fullsize models that are not super-sensitive, you'd probably be just fine with enough gain to achieve full output (2.5 µV of noise + 12 dB is 10 µV or 2 dB SPL into something 102 dB / 1 Vrms). Likewise, it has to be made sure that PGA output level is well-mapped to headphone amp output level. Thus depending on where your balanced signal is coming from, the balanced receiver would have to have a gain of -6 dB to -12 dB (maybe -14 dB).
Make sure your balanced receiver can do less than unity gain, since INAs often don't - common balanced line driver circuitry has 6 dB of gain, so you're generally at least that much hotter than unbalanced, and pro levels could be up to +22 dBu (almost 10 Vrms), while the PGA's input is somewhat limited by its +/-5 V supplies, so you can't run more than 2.5 Vrms into that. Reading Bruno Putzeys' "G-Word" whitepaper is advised. +1 for converting to unbalanced and back.
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