Levels are even harder to match if FR is different. Vinyl is challenging in that way. Pre-amps, carts, platters, weights, alignment, etc. are all going to have gross (easily measurable) impact on sound. It gets to be a cluster-fuck because the synergy thing does get to be very important.
Most people here are quite experienced and would be adjusting volume on the fly to suite their desired listening levels. At least I do. Nope, not scientific. Besides, I don't subscribe to the "louder is better" claims, at least with experienced listeners who have control over the volume knob.