This sounds like a problem with your impedance. If you are using the yellow wire on the secondary of a weber WSE15, then at 8 ohms, your output tube is seeing a 5000 ohm load. At 4 ohms on the yellow secondary, the output tube is only seeing a 2500 ohm load. Also on the Two Stroke, your negative feedback loop is tied to the impedance switch. So if your impedance is incorrect then so is your negative feedback. You say that the 2×8 cab is 8 ohm, but then that would mean that each speaker independently is only 4 ohms if they are wired in series, or 16 ohms if wired parallel. If so, then using just one out of that cab will be a different impedance than using them together in the cab. Another problem is that I see your other secondaries running through your amp still. once you are done with how you want your impedance wired, you should cut back all the unused secondaries off your output transformer, so they will not cause feedback issues.