I never really bought into the philosophy Naim demanded in terms of upgrading boxes, I went from a 160 to an Avondale many moons ago. That Avondale ended up being loaned out for 12 years, I recently got it back and upgraded it with new caps, Shottky diodes, dual transformers and Qudos output modules - essentially a new amp. Compared to the previous NCC200 boards in the amp, which in my view still had vestiges of the Naim sound, the newer boards have a much more open and revealing sound, but also retain the driving power and grip. In those interim 12 years i've had many amps including ARC, Audio Innovations, Hypex, Threshold and a few others.
I would say in this form you would have to make a very big investment to improve it. It actually sounds more like a really good Class A amp now.