I have a SR5005 as my home theater amp. IMO, there are two issues to consider when using it in the way you propose -- DAC and headphone amp.
First, the DAC. IMO, the SR5005 does an excellent job with D to A conversion. I use an Apple TV via HDMI and have also used my iPod Touch via the optical out of a Pure i20. I think you would be very happy using your MacBook Pro's Toslink out, but for your iPod Touch I would suggest something like the i20 or a Wadia dock instead of the headphone out or a LOD. This would let your SR5005 do the D to A conversion, rather than the iPod itself.
Next, headphone amp. IMO, the SR5005's headphone amp is not very good. No surprise, since almost every modern receiver considers the headphone jack to be an afterthought. Easy to drive headphones like Grados sound OK, but harder to push cans like my T1 and LA2000 Lites are not a good fit. I don't have Sennheisers anymore, but based on my past experience I think the HD600 would probably sound dull, and the HD650 wouldn't be good at all either. I think you'd be best served by hooking a dedicated headphone amp up via the SR5005's tape loop. Something relatively inexpensive like a Matrix M-Stage or a used Purity Audio KICAS would push either the HD600 or HD650 much, much better than the SR5005.
Bottom line: yes, do get a headphone amp, but don't worry about a DAC/amp combo, since the SR5005 does the DAC part just fine.
Hope this helps.