Yeah, since the spec sheet states MM sensitivity of 2.5mV, and the line stage states sensitivity of 0.25V (= 250mV), that is a factor of 100x which is 10x * 10x = 20dB + 20dB = 40dB. That’s assuming the MM output pipes into the unbalanced input line path. Typically input sensitivity means the signal level required to produce full rated output.
I’ve used SUTs configured to produce 10mV into the MM stage (even up to 12mV once, I think). It can in fact sound great. You’ll lower the relative noise floor this way, but you are cutting into the MM stage’s overload margins. Sometimes I’ve noticed a bit of harshness when pushing the MM stage like this. If the MM stage is solid state, you might have lower margins than with tubes (but it really depends). As others sad, it’s a give-and take compromise. In *most* cases, you want to shoot for 5mV. But if your system is lacking line gain and you have to really push the volume to get where you want, you might prefer the 0.4mV on 24x tap - and that’s fine! In "most" systems, the 12x tap would be more ideal.
Also when doing the calculations, higher step-up ratios are not without losses - due to loading (Ohm’s Law). The losses depend on the coil ohms. So for example, if your 0.4mV cart was 12 ohms (e.g. Benz Glider, Wood), it sees a load of 82 ohms when used with the 24x taps (82 = 47000 / 24 / 24). Then the loss is calculated as 20* log( 82 / (82 + 12) ) = -1.18dB. So you lose just over 1dB from the load the cartridge "sees". Losses are usually bad, but this -1dB actually helps you on the overload margins.
The Quadratic is a nice SUT! Really sweet bass, very clean midband, slightly relaxed top end.
Just one more note - having ample overload margin is so important because the peak levels off the record can be much higher than what the rated cartridge output indicates. I don’t have any measurements here but IIRC you can expect at least 6dB (4x) higher than the rated output, and possibly more. At 10mV (calculated) and MM overload at 80mV, that's a factor of 8x which should be "ok". Of course, the overload is frequency dependent ;)