My opinion: your macs have great audio outs. The Asgard is a great addition and if you want to buy a DAC you can do 2 things:
1) buy the fiio D3, costs you $30 and does a fantastic job. It features Wolfson 8805 chips to convert optical spdif to rca that you can input directly into the Asgard. I have the D3 and it rocks, couldn't find meaningful difference to the icon HDP (for nearly $500) as DAC or the E7 as DAC. Your iMac and MacBookPro both have mini-toslink optical out (which gets automatically activated when you put a mii-toslink cable in the headphone out). Voila!
2) buy the fiio E7 plus L7, with that you can utilize the E7 Wolfson 8740 DAC and connect via USB. The Amp on the E7 is good but not great. Plus you can use the E7 as combined DAC/Amp when you are traveling with your MBP. Costs $110
You can use the E7 as well as portable DAC/Amp when you travel with your MBP.
I would prefer 1) the optical connection is great as it disconnects any powertransfer from Mac to Amp. And it's dirt cheap as well.
If you can try more expensive DACs and IF (and thats a big IF) you find quality differences, you can replace the D3. However I doubt it. the amp is more important in this regard...
That's exclusively my own opinion, YMMW. Am currently using the Schiit Valhalla with an D3 in the office and the HDP with Asgard at home.
I connected all DACs to all amps (Asgard, Valhalla, icon HDP) the amps made an audible difference that was quite easy to spot, the DACs did not or it was so microscopic that it's not really worth mentioning or I could even be just my imagining it.
However, you can't go wrong with the Asgard.
If I would have to re-buy any of my gear I would buy exactly this setup: Asgard and D3. Done. $280 plus tax and shipping. Maybe I would upgrade to a Lyr with the money I saved on the DAC :-)
Edited by Koolpep - 7/1/11 at 2:20pm