As you rightly guessed the Project Headbox II is not much better than your average integrated amp's headphone output. The Project is really designed for providing a headphone output when none is provided, e.g. on a TV or amp with no headphone socket.
I'm afraid I don't know how good the headphone output stage on the Arcam A32 is, so it's difficult to say if the
Graham Slee Novo would be a big improvement. The Novo is an great little amp.
The
Graham Slee Solo is the reference that other amps are judged by, so if budget can stretch that far is definitely worth considering. I would be very surprised if the Solo was not a significant upgrade from the Arcam A32.
At HiFi Headphones we do offer all out amps on 14 day money back guarantee, so we'd be happy for you to try any of the amps out and if it didn't work out you could return the amp for a full refund.