You've probably experimented more in that area than I have, so I'll bow to your greater experience, but whenever I've tried it, I've never been happy with the results of using a non-Naim preamp with a Naim power amp, or vice versa, providing one wants to hear what the likes of a NAP 500 can do, when used in its intended sonic environment.
The only Naim components I've had consistent success with using outside of Naim systems, is their CD players, which are superb and can easily slot into many other types of high-quality systems, as indeed is the case with those from Rega.
However, I'm not saying that Naim preamps or power amps can't be bettered - far from it - otherwise I'd still be using my old Naim system, without having moved on considerably from it with the system I've got now!
My advice to Mike would be not to make any changes to the heart of his system until I've visited him and brought down my modded Croft preamp, which we can then try with his NAP500, and he can judge the results for himself. Trust me, if that doen't work with his power amp, and considerably improve on his current preamp, then I doubt that any other Croft preamp will.
Mike, I'm hoping to get down to your place sometime early next month
Marco.