Headphone Amps with left/right volume control???

I think you are unlikely to find commercial amps with this feature. I would suggest you approach some of the well regarded DIY builders like voodoochili who post on head-fi.org. They might well be able to build something like this for you, and there are some very highly regarded DIY designs.

i would recommend you call , john grado at grado labs, he might be able to make you such an amp for youe needs, hope this helps!

There are many balanced amps with separate volume controls for the R and L.

WWW.Headamp.com

You can get a GS-X headphone amp, it has balanced volume controls.

Do not go through Grado, they aren't great amp makers. The little Reference amp is not even that much better than a Pimeta DIY amp which costs $50.... yet they charge $500.

Headfi will have more than you'd ever want to know about headphones.

singlepoweraudio.com
headamp.com
either of these will be able to help you out.Both offer great headphone amps, tube or SS.