Thanks Serge, the Arcam is an Alpha 10 which I used before I got a separate
pre-amp, amp setup. The missus tried a NAD 320BEE before the Arcam but said
the boost was not enough. The NAD is now in the office rigged to the TV. The
Arcam is so big taking up a whole shelf plus it gets quite hot even with the
speaker switches turned off. Let's hope it's a cool summer!
Although you may consider the Arcam amplifier to be a waste, it being used
purely as a headphone amplifier, I think it is probably the most efficient
and sensible way of achieving what you need. I too can't think of any
headphone amp with a balance control, so the amplifier route seems the
most sensible. You don't say which Arcam amplifier you're using, but if
it's an Alpha, what they're fetching on EBay isn't so far removed from
what a headphone amp alone would cost, and we've already established that
won't do the job.
So, my advice is use your Arcam, or buy a cheaper amplifier on EBay which
does the job.