This is just a suggestion & it'll cut out a load of digital/analogue conversions
Why not get a USB to S/PDIF interface & then making a small transformer on a ferrite bead mount this inside of a decent phono plug & run a balanced 110 ohm digital line to a Behringer DCX-2496 as this will accept a digital input of up to 24 bit & 96Khz. The DCX is a digital crossover which then converts to analogue on up to three outputs per channel.
You'd then need a single 6 way ganged volume control or stepped attenuator for volume control. It keeps everything in the digital domain up to the outputs of the DCX crossover
DCX-2496 Ultradrive Pro