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. The 3-wire serial control interface allows for connection to a wide variety of host controllers, in addition to support for daisy-chaining of multiple PGA2311 devices.+1 for converting to unbalanced and back. The PGA2311 also provides the capability to drive 660-Ω loads directly without buffering. The PGA2311 uses an internal high-performance operational amplifier to yield low noise and distortion. The PGA2311 device is a high-performance, stereo audio volume control designed for professional and high-end consumer audio systems.
parametric-filter Logic & voltage translation.
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