Wasn't familiar with Graham, but with it being a BBC design, I would expect the KH310 to sound better.
The 5/9 is a very distinctive sounding and measuring box with a definite non neutral balance. No idea why the larger 5/8 turned out the way it did, but the 5/9 is this in (kind-of) miniature and was designed to suit the BBC and them alone although other broadcasters used them in editing suites as my pair were. Apparently, none were sold domestically until the BBC had taken what they wanted. Look at the Harbeth M30 from nigh on twenty years ago as reviewed here a while back. The balance is smoother than any 5/9 version (the Graham still has the 3kHz peak and sucked out upper mids, where the derived M30 has a straight-line downtilt). There are two UK speaker firms I can think of which actually do a lot worse in terms of response and UK audiophiles don't seem to care, so there you go.
If you wanted a similar size passive speaker with a FAR superior performance (you ought to see how 'soft' and easily deformed the polypropylene 5/9 cone is), I'd take a look at the Harbeth C7 in 'XD' form - or closer to home, the M30.2-XD. The former knocked me flat how much better it seemed to be over its predecessor and th elatter is a further very subtle tweak on the original 30.2. Amp was a Hegel 120 so nothing outrageous, but seemingly very well designed and performing. I'd suggest ther fC8-XD/Hegel combination could almost be regarded as a 'relaxed monitoring system.' I expect the Neumann's would be better in a forensic monitoring sense but I'd love the chance to see if I could listen to them for hours on end in a domestic environment..