Interesting, I have a C2700 and i'm thinking of getting a better phono stage. I'll follow this thread.
I will say though after a tube roll the built in ones are much better. the JJ tubes are not the best soudning tubes IMO. I have yet to find anything that was significanly better under $4K so you maybe in the not better but different area. The pass will most likely be better in some areas like bass control, but less in the midrange and sondstage. tube-SS thing more then anything. I've not hear the pass though so your experiance may very.
I also think the Mac's built in stage is rather easy to change settings/loading etc.