The simplest way, the way you've been doing, is just headphone jack to aux input on the amp, and to be honest, the Macs seem to have slightly better headphone-outs than most laptops. Another way is to use a DAC. This is like an external soundcard for your computer, where a more rigorous (and complicated) circuit converts the 1's and 0's of your music files into an analog wave (the wave that travels through your RCA/headphone cables, gets amplified by the amplifier and then pushes the speaker diaphrams back and forth).
Entry-level external DAC's tend to start around the $100 range, usually run off of usb-power, and only have 1 or 2 input options. The sky is the limit as far as how much one can spend on a DAC, but as with everything else, the law of diminishing returns is present (the difference between a $300 DAC and $400 DAC is SUPPOSEDLY much less than the difference between no DAC and a cheap one, I say supposedly because I haven't spent that much on one myself yet.) Generally, results of using a DAC are clearer transmission, crisper sound, more "transparency", and more fine details in the music, along with better soundstage (the positioning of the instruments and noises within the mental "room" you hear them in).
Some common "entry" level choices are: uDac2, the ODAC, the Music Streamer II. I have a Gamma2, which is a Do-it-yourself DAC that gives you a few more options, but unless you're familiar with circuit-board assembly, I would stay away from that, as it's a fairly complicated circuit with small components. I also had the uDac2, which was nice, but was more of a DAC+headphone amp, and the headphone amp couldn't hold a candle to my vintage stereos. The HRT Music Streamer 2 gets very good reviews and is no frills, usb goes in one side, RCA connection goes out the other. They're ~$160 new, but show up in the used section here fairly regularly. (I buy all of my equipment used, audio guys tend to take good care of their stuff, and if I ever decide to get something different, I can sell it for nearly what I paid for it, and not lose much). The Audioengine D1 may also be worth looking into, at around the same price.
Note: Keep in mind, these won't work with your iPod, only with your laptop. iPod USB connections have special tamper protection circuitry that basically means that only certain "licensed" products can access the stream. The real kicker? Most iPod DAC's won't work on computers! The only entry level iPod DAC's I can think of are the iDo by nuforce and the HRT iStreamer, which is the same as the Music Streamer 2, but for ipods/ipads. These are both in the $200 to $250 range (the licensing and chips cost extra money for the manufacturer)
May I ask what Sennheiser headphones you are currently using though? It's possible that a headphone upgrade might be more bang-for-your-buck than a DAC upgrade, given that the Mac's headphone out isn't as dreadful as some, and you've already got a solid amp. (I just looked up the PM710, and I'm a bit jealous, that's a mighty fine stereo you've got there. I've got a Marantz 2245 myself)
Edit: Just saw you plan on sticking with what you've got now, there's not a thing wrong with that, I hope someday I can do the same!
