So yeah:
- For a typical fixed 47Kohm MM input, a 40x SUT gives your cart a 29 ohm load. You CAN’T raise that 29 by paralleling anything. If you try to add resistance in series, you will lose almost all of your actual signal before it hits the MM input (voltage divider). As mentioned earlier, you have to either modify the phono stage to get higher than 47K input (600K = 375 * 40 * 40, as mentioned ealier) or switch to an MM stage that has non-standard loading options above 47K.
- Rogue Ares has an MM option for 100K and 1 MEG ohm loading in MM mode. Phono stages with this kind of option are few and far between. 1 MEG would give you 625 ohms off a 40x SUT (1 million / 40 / 40). 100K is 62.5 ohms. Still, you target 375 eludes you.
- A 1.4 ohm cart will be FINE into 29 ohms. Like lewm, I don’t see much value in going higher. I don’t hear much difference in small gradiations of load, at least until you’ve gone way too far (low ohms) for the coils at hand.
- Your 1.4 ohms screams "My Sonic Labs". If so, these carts also have high outputs - like 0.5mV. A 40x on this (20mV) is going to push you way towards the overload margins of most phono stages, which will likely have a very negative sonic impact - much more so than your loading concerns.