That's a lot of power for a person standing so close, but I like it
I don't have an alternative like you're hoping... but I hope this helps.
I used a 2.1 computer speaker setup for mine... it solves the volume control issue as it comes with a knob.
You can use a standard car audio amp and a regular 12VDC power supply (available at any old electronics shop). There'll be 2x positive terminals on the amp (1 for power, the other for "turn on"), you 'could' bridge them, but I think it would be better to turn the amp on using a 12V+ from the PC power supply, it won't drain power from the PC's power supply this way, but use it like a relay to activate it (so it's only "on" when your computer is on).