Some source questions...

My current setup is a Sony NS-975V DVD player and a DacMagic. I will be getting a Sony XA5400ES to replace the DVD player (and free up a connector on the DacMagic).

When doing comparisons, will the 5400's digital out -> DacMagic provide similar sound quality to the NS-975 digital out -> DacMagic? Or, would I be better off trying to sync the playback of the two players when doing a direct comparison?

Are the HD-650s/Sony MDR-SA5000s revealing enough to give a good indication of the differences between the sources? I plan on getting a high-end headphone eventually, but probably not for the next 6-12 months. I would get the headphone first and CD player later, but the 5400 is apparently rare, and as far as I can tell, easily one of the better values in high-end.

Finally, how would the Matrix M-Stage compare with the built-in headphone amp of the XA5400ES?

Ok, now that I actually have the player...

I tried A/B'ing a few tracks (XA5400 straight, or through the DacMagic). With my current headphones, the improvement is slight, but definite. For example, bass guitars have a noticeable fullness that they lacked on the DacMagic. I haven't yet had time to do extensive comparisons.

Comparing the XA5400's onboard amp and the M-Stage... The M-Stage wins hands down. I'm definitely keeping it hooked up in my main setup.

Though the biggest thing: No more juggling inputs: My PS3, ND-S1, and laptop are all now happily coexisting with the DacMagic, and the CD player is on its own.