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

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.

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.

Thx @lewm - you make some very good points.  

I have a MC cartridge and the Accuphase phono stage has MC load settings that are selectable from among 4 values “10Ω/30Ω/100Ω/300Ω” on the front panel of the pre-amp. 

I am sure that the optional Accuphase phono stage is well designed and built to a high standard, and it should work well.

Thanks to all for your advice and input. Here is my update:

I installed a current production Accuphase AD-2900 phono unit into my C-2450 preamp. It is a substantial unit that is very well built, was easy to install and works flawlessly.

The preamp has controls on the front panel for selecting the phono stage, choosing MM or MC, and varying the MC load from 10, 30, 100 and 300 Ohms. It all works seamlessly.

It is hard for me to compare it to my previous set-up as I am using a different amp and preamp in addition to a different built in phono stage.

Suffice it to say vinyl sounds wonderful - very musically rich, detailed, fluid and engaging.