Have you looked at a headphone preamp, like the Creek OBH-11 ? (This would connect in the tape loop of your pre-amp - would be even better if you have a spare tape loop). I think it's about $200 .. a lot cheaper than changing your Quad setup.
I have no idea if it is any good, but Creek products tend to be quite well reviewed.
Sean
I have no idea if it is any good, but Creek products tend to be quite well reviewed.
Sean