Agree with spritzer. Now the original 6SN7 is the octal equivalent of the 6CG7/6FQ7 in the SRX Plus, but with only 4 tubes, they are pretty much limited to the Egmont topology, especially since they had both single-ended and balanced inputs. In fact, Jason Stoddard says, "It’s not a unique design—you’ll also see stuff like this also from John Broskie," Here is Broskie's stat amp. You will see it is very similar to the Egmont posted above, with a wrinkle - specifically, the resistor/cap combo in the input diff amp tail to do some Aikido style power supply noise cancellation. If Schiit was running the power supply at +/-400 volts I hope they were using the A/B variant of the 6SN7 tubes as that is over the rated voltage of the 6SN7.
Note that the SRX Plus uses 6SN7GTA/B (max static plate voltage 450 volts, max power dissipation 7.5 watts combined) which increases the voltage and power limits a bit over the 6CG7 (max plate voltage 330 volts, max power dissipation 5.7 watts combined) in the original SRX, or the 5965 (max plate voltage 300 volts, max power dissipation 4.0 watts combined) in Broskie's design. Using the A/B variant of the 6SN7 in combination with the constant current sources gets the circuit off its knees in terms of drive capability, although it is not nearly as capable as the top Gilmore designs in ultimate drive capacity. Incidentally, if anyone is concerned with running 6SN7GTA/B tubes at 320-350 volts static plate voltage, RCA has plate curves running up to 650 volts plate voltage. 400 volts on the other hand may be pushing things a bit.