MC Load Resistance

Lewm, just a general guideline I have found very useable and giving very close to perfect results: choose a transformer which primary is about 10x to 12x the source impedance of the moving coil. Go for a step-up ratio not higher than 1:12. The resulting transformers primary will give a very good matching inductance (and resistance....) to the given moving coil.
The basis has to be a SUT which prime design goal and feature is extremely wide frequency response and extremely little derivation from phase.
Try to restrict yourself for the above recommended step-up ratio - its a kind of "royal device" when going for the best matching in moving coils.
If your source impedance is rather high - say 20 to 40 Ohms (some DL-103 clones etc, Koetsu Urushi, Clearaudio etc.) - do lower the step-up ratio to 1:4 or 1:6.
Always go for quality and low step-up ratio in SUTs - NEVER go for high "gain". The majority of the gain must always come from the phono stage itself.
The extra gain from the SUT shouldn't be as much as possible, but as little as possible.
I do use a SUT always as a matching device in the first.
The extra gain is a bonus to me - certainly not the core issue.
Thanks,
D.

Lewm, FWIW in working with the Jensens we notice more degradation when they are set for the higher stepup ratios and almost none at the lowest stepup. (Jensens have multiple windings that allow a variety of combinations for different stepup ratios.) The lowest possible is usually sufficient for most preamps (1:4). While I don't agree with Dertonearm (the effects of the transformer degradation are easily heard, at least on our gear), his recommendations otherwise corroborate what we have found.

Lewm, as you are one of the true audiophiles who do NOT settle with what the industry put in the package they bought (you have (have had) parts custom upgraded and modified the amplifiers you use), let me suggest you give it a try with your modified Atmasphere MP-1 and a Jensen or Lundahl transformer.
The Jensen 347-AXT can be used with the Urushi and the Lundahl LL-1931 or 1933 can be used with the Ortofon 7500. However none of these is really ideal suited for a VERY low source impedance (and thus inductance....) LOMC while offering a decent (1:8 to 1:12) step-up ratio. If you want to restrict yourself to one SUT first, I would go for any of the two Lundahls mentioned first - they do offer primary options which can accommodate all your top-3 cartridges (vdH Colibri ...).

In SUT - as in cartridges/tonearm combinations - there is no single "BEST" SUT. There are superb matching combinations - and combinations of great individual components (MC and SUT) which do NOT match.
Its again applied skill and knowledge to get the best possible performance.

If you give it a try, I'd really look forward to learn, if you too do hear the "degradations" Atmasphere (..the human person - not the preamp....) hears with his MP-1.
Select the best (read: .... widest frequency response, close to perfect phase = most expensive...) SUT, settle for low step-up ratio, use Lundahls or Jensens recommendations or spread sheets to careful fine-tune the secondary resistor in accordance with the phono input resistance and try to meet the above stated matching rules.

I bet it will proof worth the effort.

Lewm,
+++ ... Axel, are you saying that you now like a 47-ohm resistor on the primary side of your SUT? I presume you also have a 47K-ohm resistor on the secondary side. Is this correct? +++
No! please see below.

A 30dB SUT (1:31.6 ratio) has a "natural impedance" of 47 ohm when going into a 47k secondary input impedance (the phono-pre).

Since the primary impedance = secondary input imp./ ratio ^2 = 47k/31.6*31.6 = ~ 47 ohm primary impedance.

So what I now like is the 47 ohm primary *impedance* of the SUT without ANY resistive loading (other then the negligible 1.1 ohm of the primary coil winding itself...)

I hope this should clarify the matter.
Greetings,

Thanks, Axel. I am comforted to know that you like that set-up, because I never could quite understand how a 13-ohm load could be superior to any higher value resistance, using your cartridge. 47 ohms is more in the ballpark of what I would guess would work well.

Dertonearm, thanks for the tip. Unfortunately or fortunately, I modified the phono input stage of my MP1 to use an MAT02 bipolar transistor as the bottom half of a cascode, where the top half of the cascode is now an ECC99, in dual-differential configuration. This, in conjunction with the output stage gain section, gives oodles of gain, even for the Orto MC7500. (The MAT02 has a Gm of 400!) And I am very pleased with the sound quality. I guess I could use a VERY low step-up ratio SUT, just to test your hypothesis. And there are tricks I could use to reduce the gain at the input, as well, without changing the topology. (I have to do this when I use the Colibri, which puts out 1.0mV.)