NZtechfreak: Definitely check out Computer Lounge, they will have most of the those to audition (they did stock HiFiMan previously, but last I checked they weren't listed on their site any longer). Also, there will be a headphones meet later in the year where I would hope almost everything will be available for auditioning.
For the HD800 you will certainly want an amp upgrade, and even the others (including your HE-500) will sound better from a more powerful amp.
As an aside, I have some PM-3 coming to me now to compare to the Senn Momentum 2.0, they are basically the same price so a good head to head comparison is needed!
Thanks. I'll contact Computer Lounge and ask them about this. Where is this headphones meet?
As for the amp upgrade, that is something I've researched this a lot and I get a lot of mixed responses about this. I've heard and read a lot of criticism about tube amplifiers and how they add distortion and color to the sound. I've asked the guys at the audio forum on linustechtips.com and most of them have told me that an amp upgrade isn't necessary for these headphones.
Anyway, thanks for the advice. Maybe an amp upgrade is something I'll consider later down the line. Any you can recommend?