Tough one. Digital attenuation with hardware volume control and pro level digital out.

If it were me, I would take the easy way and call my man at Sweetwater, Ryan Clapper. And I would doubt there is something that perfetcly fits your needs, you will probably end up with more features than you need, like a mixing desk or lots of i/o. But htere are a few master volume knobs intended to work with DAWs, that might be usable.
Perhaps there is a digital monitor controller with AES outputs (maybe on Mars?

) DSP monitors are newish, so the demand to feed them digital signal is not real strong yet, but it's coming.
Look at the pro monitor mfgs who make digital monitors like JBL and Genelec for hardware controllers for their monitor systems. Like JBL Intonado, or whatever it's called.
Pro Tools, etc probably have something like that but you don't need all the i/o that come with it. There are many analog monitor controllers available.
All modern DAW software use 64 bit DSP, so you could use an AES output card like Lynx, with a 64 bit player/dsp host, but you need to find one that can be controlled by a hardware volume knob. The professional DAWs have MIDI input, I'm sure they can control master out. But the DAWs aren't ideal for consumer music playback, picking songs and making playlists, streaming internet radio, etc.
JRiver is a nice consumer focused music player with excellent 64 bit software volume control and 64 bit VST host, but I don't think it has any provision for a hardware volume control, I don't know. If your application can tolerate using an iPad to remote control JRiver volume then it might be the wtg in the consumer realm. Then just find an AES output card for the PC.
/end stream of consciousness...
