V1a consists of a cathode capacitor C1 that bypasses cathode resistor R2. Therefore the initial input stage is not AC feedbacked To reduce the gain I'd remove C1 or at least provide a serial R that feeds C1 to ground. May be 2.7k for cathode resistor R2 helps to enlarge headroom capability. I'd assume the grid voltage to around -1.2 .. -1.5 Volt. Max. input swing around +/- 0.7 Volt peak to peak at most. IMO 1 Volt rms input level will easily exceed the negative grid voltage and therefore start to distort the signal due to grid current. An active electronic is capable to drive signals of +/- 4 Volt peak to peak. I have never seen of standard cathode basis stage that can cope with those strong levels at the grid, independently which tube is stuck. The design obviosly intends to yield extended even Harmonics even on smaller input signals due to bypassing R2. R2 reads to 1.5k and very likely the bias point is close at the edge of asymmetrical output biasing. IMO the design is very well sophisticated if you look for a bluesy sound, but in my experience a schematic design like that must not sound "clean" with enlarged headroom. I'd remove C1 and would try 2.7K for R2. Disadvantage will be that different tubes don't hit its characteristics into the overall sound of the amp. The input stage will sound more linear, deminishing a little bit the different characteristics of different tubes.