Basically your right on, all you need is a toslink in on the amp. I've been thinking about doing the same thing, except my Mac doesn't have a toslink out
So I'm using an old receiver with bipolar front speakers stolen from my big home theater system and a big subwoofer. Just using the RCA audio outs which work fine, since I only listen to iTunes and don't have games or any 5.1 surround sources that i want to watch on a 19' monitor. I've got a major home theater in the front room with 4 KEF 104.2's and Definitive centers and a 15" powered Velodyne sub+ a bunch of amps with a Yamaha 5.1 receiver.
A good set of bookshelf stereo speakers and subwoofer is 80% there, the surrounds just add that little bit of envelopment in the sound field, which is nice if you have a 5.1/6.1 media source.
Paying more money for a receiver will get you bigger amps and or connections to add additional amps and input sources later which is nice, sounds like you don't need anything else.
Instead use the speakers you have and set then to small and direct the base to a hopefully powered sub. Most of the power required to reproduce music is needed in the low bass range. So get a decent mid/bass range subwoofer (later) to make up for the lack of power you will get from an economy amp and smaller center front speakers. Matching speakers, sonicaly speaking, will still get a good surround effect, but to get the lower midrange bass is going to require an amp of decent size, if you want some thunder in the lower range. Larger speakers are generally more efficient than smaller ones and ported speaker (having a hole or tube between the interior and exterior) are more efficient, putting out more sound, than totally enclosed designs. This is more important with speakers containing larger, say around +4" sized drivers, or where mid bass frequencies are capable of being reproduced.
A couple of tips. Try to set the tweeters at ear level. Use the test tone to get an equal output from all the speakers. Use a piece of colored tape to designate one wire positive. This can be the smooth wire, the gold colored conducter, the one with writing...etc. Just stay consistent throughout the, wire to device, to speaker hookups. Nothing will get hurt if it's out of order, but you might lose some bass frequencies due to cancelations, if one speaker is pulling and one pushing. I also code right and left wires with some colored tape.
Be VERY careful not to let wire strands from one wire touch the other, especially at connection points. it pays to solder the ends or use special connecters to avoid electrical shorts and blown fuses or worse!
Set the speakers in as symmetrical pattern as reasonable, the rear speakers can be mounted a bit higher, it's easier on a back wall, rather than out in the middle of a room.
Don't turn the rear speakers up too much, they shouldn't be noticed unless some specific sound effect, from an aggressive soundtrack stears the sound there. Usually the purpose is to get a sense of envelopment, not to have the surrounds up so high, that you would get distracted from the action going on in the front speakers.
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dave