Hi Reb -- well what I know about cartridge loading, I know mostly from experimenting with my vdH's over the years. The required load impedance (or preamp input impedance) for any given MC cartridge is generally a function of its internal coil resistance. The 0.5mV Colibri, for instance, with a coil resistance of approx. 30 ohms has a required load impedance of "greater than 200 ohms" (which presumably means 200 ohms up to infinity!?) but then, they do specify an "optimum" load of 500 to 1000 ohms (which is why I always used the 825 ohm setting on my Levinson phono preamp.)
Now, the Transfiguration folks are even less specific than vdH, (greater than 9 ohms? duh!) but after looking at the specs for a lot of MC cartridges, I've come to realize that the specs for cartridge loading vs. coil resistance are pretty much proportional across the board, for example: The average optimum load for the Colibri described above would be 750 ohms (halfway between 500 and 1000) If you divide 750 by the coil resistance (30 ohms) you come out with a multiple of 25. OK the internal resistance of my W is 6 ohms (Isn't it amazing how the Temper W gets about the same output as a Colibri with what appears to be only 20% the number of coil windings!) OK, so if you multiply 6 ohms times our multiple of 25, you come out with 150 ohms as the optimum loading for the Transfig W. For a V, at an internal resistance of 3 ohms, the optimum load would be around 75 ohms.
Now none of these numbers are cast in concrete, however, they do represent a fairly solid point of reference, and will apply to any step-up device, whether ss, tube, or transformer. Assuming the cartridge is set up so the stylus is in reasonably correct relation to the groove modulations, and taking into account some other minor sonic variables (like your speakers) you shouldn't have to deviate more than, oh I'd say 50% up or down from the theoretical mid-point to find the best response. In the case of the Temper V, that works out to somewhere between 40 ohms and 120 ohms.
There's something else I've noticed sonically in loading MC cartridges. Let's use the Temper V as an example. There is never a huge difference, sonically, between the top end of optimum (120 ohms) and infinity (lets say 47K ohms) so it's sometimes easy to miss that high optimum point because everything above it sounds about the same. But not below the low optimum however! That sounds like crap! This is why it's best I think to start at the low end of optimum and slowly go up until things start to sparkle, bass tightens up, and then stop! That is the point where you will get the most output and flattest response across the frequency spectrum. Right now, I have my Temper W looking at a 100 ohm load which is one of the preset loads in my phono preamp. It's probably a little low, but I'll leave it that way for now. Maybe in a couple hundred hours, I'll get off my ass and try 150 - 200 ohms using some really good Vishay resistors. But no higher. I already tried 825 ohms when I first installed the cartridge because that's where my preamp had been set for my vdH Frog, and it sounded pretty screetchy, almost unlistenable.
A long time ago, I had a vdH TWO cartridge. It had an output of 2.25mV! I had an ARC SP-14 preamp. It had a really great tube (lo-gain moving magnet) phonostage with a built in standard MM load impedance of 47K ohms. The internal resistance of the TWO was 96 ohms, so the optimum load should have been about 2400 ohms. I put in some shunt resistors to drop the impedance down (I didn't solder them) Well, it did get just a tad louder, but it didn't sound that different (to my ear at that time.) I decided not to mess with it. But had I known then what I know now, I would have dialed it in!
I guess my main advice, based on what I've learned over time, is don't paint all MC cartridges with the same brush when setting them up. How your previous one worked best is not necessarily how your new one will work best, even though they may have similar outputs, VTF, etc. In my opinion, the most important things to set properly before attempting any fine tuning, are VTF, SRA, anti-skate, and load impedance in that order. Once those parameters are correctly set for a specific cartridge's electrical and physical properties, I believe very little additional fine tuning will be required to reach the very best performance.
In fact, I would suggest listening for a 100 hours at the initial settings, until the cartridge is broken in and you are really used to it's performance as is. Then, at least, you'll be able to better tell what effect any changes you make are having, better or worse.
