One solution, which I'm presently using for my computer (2-way bookshelf speakers, as opposed to dinky self-amplified computer speakers) is to use a cheap "class D" amplifier. I have a Sonic Impact 15W 2-channel amp (A/C adapter not included). All said and done, you spend less than $100 on the amp plus wall-wart. These class-D amps (Sonic Impact calls it "class T" but it's the same thing) are really just switching power supplies, like what shows up in your computer. They're very power efficient and would be fine for your application.