As an owner of the ZxR for the past couple of years, I love it.
I have mine hooked up to some Sennheiser PC 363D's and a pair of JBL LSR305's (don't have the space for the 308's). The sound quality is awesome.
I haven't come across the AE-5, so I can't attest to that at all. I have a hard time imagining that it wouldn't be more than sufficient.
Let us know how it turns out between the two.
On a side note, I have the Z in my older son's computer and it really is very...