The PS Audio P300 is not a transformer type conditioner, it is an ac regenerator. It converts the AC from the outlet to DC and then back to AC.
If your AC power is clean with no fluctuations in voltage and with little noise, than a power conditioner or power regenerator will have less of an effect than if the power is poor. In my case, the P600 made a tremendous difference on my TV (blacker blacks and less noise) and my stereo (more soundstage depth), even without using any of the multiwave settings. But again, every situation is different.
Tube equipment is inefficient and thus the P300 would be unlikely to provide enough current over time, regardless of the make or brand of tube amplifier. The p300 is best for front-end components.
I think PS Audio offers a 30 day no questions asked return policy, so it might make sense to try the unit