+1 for the direct heated triodesThe problem with the 12AX7 is that as a voltage amplifier, it has a ridiculous amount of gain. Nobody needs 30+dB of gain in their tube linestage, and having that results in people complaining about how they can only turn up the volume knob on their preamp 5 degrees before it's ear bleedingly loud. As a cathode follower, the 12AX7 can only run a scant amount of current, so if you load it down it will really struggle to operate well. This is why the Triode Corp. preamp uses a 12AX7 in parallel for their cathode follower, but quite literally any of the other common dual triodes is a better choice for cathode follower duty. If you find a preamp with 12AX7s and low gain, it may just be that it's a copy of a vintage design using global feedback, and these are not the greatest sounding circuits either.
I personally prefer a directly heated triode gain stage with an indirectly heated cathode follower buffer. I've had good results from single gain stage tube preamps, both with and without output transformers as well. I think the recommendation of the Aikido circuit is a good one, as that is well developed and well documented, but I would stick with the lower mu higher gm dual triodes in that circuit.
I don’t think I have +30dB in my entire system…and, with the right speakers, haven’t needed it.