Figure out the open-loop gain of the circuit, subtract about 20dB and there's your answer. (I say 20dB because, if you use much less feedback than that, it tends to be worse than using none at all with regard to higher-order harmonics. See Baxandall's
article from 1978).
I can't be bothered to calculate the gain of each stage at the moment, but a fairly safe bet for those two pentodes with the circuit values shown is somewhere between 5000 (74dB) and 10000 (80dB). So, with a 20dB minimum feedback factor, that's 54dB to 60dB of closed-loop gain. And then factor in transformer ratios.
By the way, a much better way to handle the gain switching would be a series string of resistors instead of individual switched resistors. In the event the switch goes open, the preamp would snap to its maximum permitted closed-loop gain rather than going completely open-loop, which could be quite alarming!
Another method, even "safer" in terms of switch failure, is to select the feedback resistor for the minimum desired closed-loop gain and connect a gain control (via a coupling cap) in shunt with U1's cathode resistor. The low output impedance of the WCF output stage makes it feasible to use a low-ish feedback resistance.
It's kind of gilding the lily, and it'd have to be done with care since it's inside a negative feedback loop, but you could get even more gain out of U2 by bootstrapping its plate load. You could do this by dividing the plate resistor in two and connecting a coupling cap between this junction and the cathode of U3A. You could also bypass U2's cathode resistor but the increase wouldn't be as dramatic.
