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

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.

My Little Luxman SQ-N150 in my office has built-in Phono, MM and MC, sounds darn good, however, no gain or impedance options for MC input. I've only used it's MM for AT440ML.

toronto, I am a proponent of "phonolinepreamps" when they are designed and built to a high standard, as is likely the case with Accuphase. From an audiophile point of view, they offer phono function without the extra chassis, which eliminates output and input jacks (RCA or XLR) and the interconnecting wires. (Also saves money, often.) They also permit a very short path between the output of the phono and the linestage, which has additional benefits regarding impedance matching. However, it is fair to point out that a built-in phono may provide fewer options for variable capacitance or resistive loading, as Elliott is pointing out. So in one sense it would depend upon how the offered features fulfill or don’t fulfill your needs as regards your choice of cartridge and any future plans you may have as regards cartridges.

On the other hand, if the phono module works fine for your particular LOMC cartridge (for example), you can always add an outboard phono in the case where you want to add a high output MM or MI type cartridge and plug it in to an AUX input.

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.