Here are Arthur Salvatore's thoughts on them:
I used the AI M3A with the built in Gold MC phono board for about 5 years. I thought it was quite good at the time. I then moved to an integrated amp and purchased the K & K Audio phono stage. Without question, this phono stage is a big step up from the phono stage built into the AI (before you ask, yes I did do substantial comparisons of the two). I then got a chance to hear the Mu Mc through my K & K and it was a significant, further improvement over the Lundahls built into the K & K. I bought a used the Mu Mc to used with my K & K. All this to say, the Mu Mc is the best step-up I have heard in my system and I cannot imagine that you would not be happy with it. However, I think you need to question if it is really worth spending a substantial amount of money for an improvement in the phono section of the AI. The AI is already quite limited and you would probably be better off spending that extra money on moving to a different preamp altogether. Just my opinion, of course.
Mid40sguy - When I had a superb prototype MM phonostage to try, I did so with a 1:10 Bent Audio TX103 with the copper option (never heard the silver unit). At that time I compared it to the Classe Audio NIL and an Ear MC-3. I now have a MC phonostage so the BA unit does not get as much use as before.
I do not know if the 3a has both MM and MC inputs. You will have to determine which gain option, 1:5 or 1:10 is appropriate if you only have an MC input. Via the MM input that is wide open at 47k Ohms, you cannot go wrong with the TX103. In my system it outperformed the other two units available to me at the time. The added convenience/benefit of it having a leadout wire eliminates the need for an additional IC. Further, the cartridge loading posts along with the resistor set the manufacturer provides allow you to dial in your desired cartridge loading.
You can also get a TX103 that allows you to switch between one of two gain settings. Happy listening!
The K & K phono stage only comes in one standard version, although Kevin Karter can customize the disign to some extent. This phono stage has both mm and mc capability and it also provides 3 or 4 load settings for mc cartridges. It cost about $1,700 new. This would be, in my humble opinion, a better purchase than the TX103 as it would give you superior phono with both mm and mc cartridges. There are other good phono stages on the used market that could be considered (eg. Audio Research PH3SE, Herron etc.).
Not answering your question, but may i 2nd, or is it 3rd, the K&K. I had mine built at $1900 or in kit form $1550. The best vinyl reproduction I have heard in my system, by a country mile. Detailed, but not harsh, great base, soundstage, whats not to like, plus Kevin is excellent to deal with. In my research on step ups, my conclusion was that the Bemt was the best currently available at a realistic price. Do remember the problem of RFI that bedevils stand alone step ups. That makes the short captive lead of the Bent a plus. IMHO, it makes a built in step up even better, step forward the K&K