Will the Raspberry Pi run Myth? Perhaps with an external TV tuner if you need TV (like the Home Run). I would hook it up to a real computer monitor though (IPS screen like the new Korean ones that are cheap) not a TV, but I am photographer and need better pixel density and color than a TV can provide. I would just use an USB audio card to get the surround sound. You could easily set up a bunch of t amps like this:
4x100W @ 4 Ohm TK2050 Class-T Digital Audio Amplifier Board 320-302
and power them on and off when you need them, they are very efficient. Speakers could be car audio ones. For me I would set up a KVM so I could connect a laptop when doing more processor intense stuff like Photoshop.