best choice cartridge for my Lamm LP 2

Frankly I would have thought the Lamm should be able to drive quite a number of carts. Nowadays its quite common to find carts whose output exceeds 0.5mv - AT 33PTG, Sumiko Celebration, Koetsu Rosewood Sig, Urushi, some Benz, Clearaudios, Ortofon Kontrapunkt h/b/c, Shelter 90X, Lyra Helikon, Titan and of course the Allaerts MC1B or Mk II. Between these, I'm sure there's one that should suit your taste and budget.

Only the low output Dynavectors MCs, Koetsu Platinums, and a couple of other hideously low output carts, like the Audio Note, Denon S1 are out of your range.

The Herron mc phono stage has 66db of gain FYI. I've used it and am very happy with it, even with carts of 0.25mv output.

Other tube stages you can consider are the Manley Steelhead, Art Audio Vinyl Reference.

The LP2 is a fine phonostage. If you like it, I believe you're on the right track to look for a cartridge that will work well with it. And the suggestion to consider a cartridge with .5mV or greater output is a good one. I'm currently auditioning a top caliber phonostage whose gain is the same as the LP2 (57.5dB) using a Shelter 901 that outputs .5mV. (This in comparison to my current phonostage reference which is a Pass Labs XONO that is solid state and offers flexible gain adjustment.) Let me just say I would be cautious about using a cartridge with a lower output than .5mV into a phonostage with much less than 56dB.

Cmk has given you sound advice and, imo, the impedence of your linestage is much less of an issue than gain and cartridge output. You might inquire of Lamm about a modification to the LP2's 40ohm load.

If you're unhappy with the Lamm, then changing your phonostage is certainly an option, but otherwise I would not look at it as the first thing to consider. If you were to look at other phono stages, as you note there are few tubed units with gain higher than 56-58dB. I am a strong fan of tube gear and, except for the XONO, my system is all tube yet I don't think having a solid state phono stage is a compromise. That's meant more as a comment on solid state phono stages as an option, not a judgement on the XONO. It is a bit surprising that both the LP2 and PH7 don't have balanced outputs.

(Sounds like you may be considering the Ref3, imo its an excellent choice for a linestage if balanced is a requirement.)

As to cartridges, the Allaerts MC1B and MC1 B MkII as Cmk mentioned, Shelter 90X, Lyra Titan, and Transfiguration W or Orpheus are a few to consider with output of .5mV or higher. You might search user systems at AA for the LP2 and see what cartridges its owners use.

Then there is consideration of how that cartridge mates with your tone arm wrt resonance frequency, which, for starters. means knowing the mass of the tonearm and the compliance of the cartridge.

Spec's can be a guide, but the best technical match is no assurance of audio happiness. At the end of the day, trust your ears and the connection you make with the music. Best of luck!

The question with the LP2 isn't particularly its 56db of gain. As Cmk and Jtimothya point out, there are many great cartridges with ample output for that amount of gain.

The real question is that 40 ohm input impedance. That isn't close to optimal for any of those cartridges, and Lamm refused to adjust it for two owners that I know.

If you like the Lamm it's worth asking again of course. Perhaps they've changed their tune.

The gain from the Lp2 is ok for nearly all kind of Cartridges. More important is the gain from the linked Pre Amp.
And forget the 40 ohm load here. I don't know why, but the LP2 sounds amazingly good with every cartridge I connected with.
I also own a adjustable Klyne 7 Phono, this one beats most out there, and I listened to a LOT, but the LP2 is a remarkable unit.
Btw. Vlad can modify it for higher Gain.