Babying or running a amp 25% or so below max output will only make a dB or 2 difference and is not a pileup breaker. Timing is a far better breaker there than shouting matches. BTW why not put you foot to floor Everytime you put your car in motion and see if it lasts as long.
When I ran a Clipperton L I calculated the efficiency and used the Hi/Low setting that was most efficient, with the lowest plate dissipation, at whatever power output level I wanted to run.
How many mistakes can you find with this discussion about SB-220 amps? BUT... a decent test to see if an SB-220 can stand a key down at SSB voltage for a quick second while loading.
I use my SB 220 on AM with a verity of vintage AM radios and always use the CW position. Way to much heat and load to run the amp on SSB mode in AM. As to the original question, I would think it is surely the efficiency of the tank or the input that is causing the lower output . Try different bands and you might find different results.
They do not mean anything on HF 99.9% of time and to suggest otherwise is not accurate. On UHF where you are generally not limited by man made noise but instead by internal noise a few dB can be a deal breaker. On HF it is mostly ego.
The amp efficiency suffers at the lower voltage due to the tube design. Run the same power on the SSB position which will require a bit less drive. Less heat means longer component life. Ive run my NCL-2000, Clipperton L/2500/DTR2000L, and other older amps in the SSB position since Ive owned them with no reliability issues.
Whatever you do DO NOT PLAY THE LINEAR AMPLIFIER FACT DRINKING GAME! you will be passed out before the power is applied to the amplifier
Carl, I must have said this a hundred times but it doesn't sink in. You'd think that you and I are making this stuff up rather than speaking from years of experience. The top band reflector had a link to a scientific test done on 1 dB increase in signal. All one has to do is see it and they will take their foot out of their mouth.