@holmz
If a single frequency SPL is measured with the same terminal voltage, that SPL should be the same for any amp.
Since almost agree that tubes sound different than solid state, then what makes up the ’voltage’ must vary. So if you measure 1kHz 2.83v at the speaker, an SS amp could be 99.999% 1kHz. A tube amp may be 99.5% 1kHz, 0.25% 2kHz, 0.05% 3kHz, etc. Additionally, depending on the amp and system, there could be PSU or loop components. An FFT of the captured output could be revealing.
There fore, depending on the equipment and pertaining to the suggestion to measure the terminal voltage to match levels, is that the program SPL may not be the same with the same measured voltage assumed to be made at a reference frequency.
Since the early 70’s I’ve had time coherent speakers: Dalhquist DQ-10, KEF 105 MkII, BiAmp Magnepan Tympany IV. From 1987 to 2020, I used Spica TC-50, which were the speakers I carried around to studios. The TC-50 was the first box speaker I heard that more correctly presented the phase coherence that I first heard on Quad 57s. and were able to correctly produce what I heard on time corrected BIG monitors in recording studios. I bought the TC-50 without asking the price or knowing anything about them as they were the first small monitor I’d heard that came close to presenting the recorded sound field. Since 2020, I’ve used Eminent Tech LFT8b’s, triamped with time alignment via miniDSP. The speakers can produce a tolerable facsimile of a square wave. And then I screw it up with tubes.