The C-2800, C-2810 and C-2820 preamps from Accuphase belong to the same family, i.e. all three are based on the same circuit/design. Furthermore, as it is usually the case with Accuphase, the improvements from one generation to the other are incremental.
IMO the C-2810 offers the best bang for the buck. One one hand, it costs roughly half the price of the new C-2820. On the other hand, it is not as old as the C-2800 (whose caps might need replacement in 5 years).
