shanek248, your receiver may or may not drive low impedance headphones any better than something like the Mixamp, but it wouldn't be a big disparity either way, might not even do that great a job to be honest...
Truthfully the headphone output of AVRs isn't always the greatest, and even when it is decent it might still have impedance issues with low impedance headphones made to be driven by portable or lower power sources.
Then on top of all that most AVR don't do surround emulation or a good job of it, which is another topic altogether from how well of a job they'll do electrically powering the headphones. I'm not sure if you were grasping those differences which is why I suggested reading the first post (expand the spoiler sections etc), if it's too confusing or MLE glosses over some concepts (been a while since I read thru it myself so I forget how it's structured) you can possibly look up some of Evshrug's guide/FAQ threads.
There are certainly some cheaper options than the Mixamp as far as surround emulation goes (DSS, Recon3D etc.), each has it's particular pro/cons... Most of them can easily drive something like the AD700, CAL, or HD558 since they aren't very power hungry headphones.
As far as open headphones goes, if you don't mind hearing the piano in the background and your wife doesn't mind a slight murmur in the background (that's roughly how she'd perceive it, depending on your volume and her piano playing etc) then they'd work.
Thanks a million. I see what you mean and don`t worry I didn`t take any offence to being told to go back and read. I will go back and take a more careful read because I don`t remember any theory in there!! Clearly I glossed over it! I did my undergrad in electronic engineering so I should be able to catch on .... eventually.
Im not completely focused on getting open headphones since I dont even know what the advantages would be in my case. I only have interest in the ad700s due to the reviews they are getting across the board for my particular needs. The fact that they are open headphones is merely coincidental. :-) This is all very interesting. Seems I have a lot to learn when it comes to headsets.
I was hoping against the odds I could use my AVR with some magical set of cheap headphones as a great surround solution. It seems this is not to be.. :-( ....lol
I will do some research into the cheaper surround emulation options you suggested but I fear I may have to settle for repairing my trittons.