When you switch to 5F1 mode you get the additional gain of the 2nd stage bypass cap, which the 5F1 doesn't have. Convert to the 5F1 feedback loop, connecting the 22K to the top of the 1.5K cathode resistor & lose the 2nd stage bypass cap.
Or, make the 2nd stage bypass cap switchable in/out (bypass the switch with a 100K resistor) and reduce the vale of the 2.7K NFB resistor to closer to the 16:1NFB ratio (680-820ohms), compared to the 58:1 ratio that the SF has stock.
Unless you are going to have 3 or 4 switches, you will need to accept a compromise here & there.
By the way, a big dB boost is normal when bypassing the SF tone stack...You could also try a 5751 preamp tube if the stock 5F1 is too gainy, but this will affect the SF mode performance too.