I agree with your reasoning and what you’re trying to do. To your original question... To see how much power your speaker is getting, so you can set the attenuator correctly, just put an ac voltmeter across the speaker terminals. For a 16 ohm speaker, 25w would be 20vac. Play some power chords and slowly turn the levels up. I have a 50w plexi and a cab with two greenbacks in it, and a Weber mass (the Mass is new, haven’t really had time to test it yet). I have a good master vol. on the plexi which lets me play at reasonable volumes. Check out Lar-Mar PPIMV mod for your plexi, they are wonderful, and pretty easy to do. They sound amazingly good even at lower levels. And imo, running an output tube at higher output levels doesn’t shorten the lifespan significantly... the idle bias is typically set at 70 to 80% of max tube dissipation.