http://www.njr.com/MUSES/series/MUSES72320.html
LM1972 is a bit less esoteric.
The LM1972 has no gain, just -0db (full volume) and down to -78db with only 4.5v to 12v supply, which it only needs in this case since it wont amplify the audio level.
MUSES does +31.5db to -111.5db.
If power supply current is not a factor, and you want an excellent digital volume, class A headphone amp with a bit of kick, I would use the MUSES, powered at full 30v (+/-15v), have the output drive 2 x 30v ultra logic level (1.2-1.8v) mosfet gate directly operating in voltage follower mode, with a big 5 watt pull down resistor to -15v (only adjusted for a 1.5-2watt draw). With AC decoupling caps to drive the headphones/speakers. Controlled by the MCU of your choice.
This makes a minimal 1 chip for gain and volume, digitally controlled, 2 opamps, or dual opamp & 2 mosfets (TO-220 packages with heatsink to radiate the 3-4 watts) for the most basic class A output, or, 2 NPN darlingtons for a tighter sound, again class A, for a nice minimal analog design. (No feedback on the output driving transistors.)
Clean output with 0 crossover distortion, but, expensive & you need a hefty +/- 15v supply.
There are nice cheaper solutions if you want to use a motorized mechanical volume control.