I thought I would muse a bit about theoretical noise performance of ribbon microphones. First, I noted above that most people are probably experiencing no better than 18dBA-20dBA.
Second, read this post by Marik:
https://homerecording.com/bbs/showpost.php?p=3540500&postcount=6
Let's ignore the issues with high ratio transformers he mentioned for a minute, and take his motor + primary figure of 0.5 ohm (round figures--also note that Marik winds very low DCR transformers himself, so that figure would not apply to his products, he is talking a typical Chinese ribbon here). He derived an effective DCR of 1.5K. That has thermal noise of -123dBV, so we'd ideally want an amp 10dB quieter than that. Thermal noise of a 150 ohm resistor (which is usually taken as the quietest possible mic amp) is -133dBV, but I think the lowest figure I've seen in real life is -130dBV (the TRP is -130dBu-A, which is -132dBV-A, or probably -129dBV unweighted). Good enough.
Back to the mic for a minute, what is its self-noise without an amplifier? If we take max possible ribbon sensitivity (without an amp) as -53dBV/Pa, then that yields self-noise of 11dBA or so, but to preserve that we need the AEA TRP or similar in terms of noise performance. Even so, 14dBA is probably more realistic for a Chinese ribbon.
Instead, if we can increase the ratio of the transformer without penalty (we probably can't, but play along), then the sensitivity of the mic increases but relative noise doesn't change any (because we step up noise along with signal). Then we no longer need a super-quiet preamp, and it's easier to do with phantom power.
I don't know the limits of that technique as I don't know much about transformer design. That's a question for Marik. Clearly my old 315 runs into problems with inter-winding capacitance because the frequency response chart shows a loss of HF at the high impedance setting. Unless Shure took into account cable losses, but I don't think they would have as that is length-dependent. Of course in a phantom ribbon we don't worry about cables, that is the point.