Thanks for the replies.
The power pack is just a simple 12-volt battery/recharger available anywhere auto parts are sold. Add an AC converter and you have a great portable power source. The problem with the A5s is that they drain the power pack very quickly. However, if you use the A2s, you can get at least a couple hours, if not several hours, before you need to recharge the battery pack. Works great for camping too.
The upgrade to the A5 from the A2 is pretty substantial, in my opinion. With the A2s, you can hear the music fine, but the A5s will give you room-filling sound. More bass, better quality bass; midrange is very good; the highs are definitely not "airy", but the information that's there is very good, and overall you just get a better sense of "oomph" from the A5s--very musical.
I own both, and each has it's application. A5s are best when plugged in to a wall somewhere. A2s can handle several hours of battery operation (using a power pack like I described above) making them extremely portable.
So, my question still stands: Given the very good sound of the iPod/A5 combo, is the sound improved by adding the 170i/DAC combo to the above or do the speakers fail to resolve the sonic improvements that you'd expect by adding the outboard DAC?