You don't necessarily need a receiver. You can get an amplifier. As long as the amp has a set of 1/4 unbalanced inputs, you can adapt a mini-stereo to RCA cable with phono-to-1/4 inch adapters and plug into the amp. Your speakers would wire into the amp. Set the amp to a good volume level, and them control volume on your Mac as you normally would.
I think the best, and most cost effective solution is to get a set of powered speakers. Unless you have a need for good reference speakers, or you just an audiophile, you don't need to get a set of speaker that require a separate amp.