slide in phono stage for Accuphase C-2450 preamplifier - is it any good?

think long, I suspect nearly everyone will recommend keeping your phono stage separate.

also, thinking long, think both MM, MC, and optional gain and separate optional loading. Perhaps multiple inputs for two or 3 tonearms, and Pass for MM

Good point - thx.  The slide in units are not inexpensive, and the choice of separates would be much greater.

But given how Accuphase is meticulous in their design, engineering, and production, I am still wondering how the AD-2850 or AD-2900 perform in an Accuphase preamp such as the C-2450.

You are very unlikely to get an informed opinion here regarding the two choices of Accuphase phono modules, unless perhaps you tell us more about them. Because I doubt any one here owns an Accuphase preamp with phono.

Fair enough - I think the two Accuphase phono stages are very similar, it is the general concept of a built-in phono stage that I am grappling with.

I have owned two Luxman integrated amps (L-509u, L-595A) that have built-in phono stages - and they are very decent. There are more compromises and challenges with a phono stage in an integtrated amp than with a pre-amp, so I imagine the Accuphase option is as good or better.