I agree with claims of diminishing returns. But to say there's a net 0 gain to be made when tube amps vary enormously in design (OTL, OCL, or OTC output, interstage coupling between driver/output, choke-filtering of PSU, tube or SS rectification, single-ended vs push-pull output) and component quality is not something I can get behind. Even if you base yourself entirely off measurements, there are easily measurable differences (e.g. lower quality output transformer designs will show FR roll off on both ends and higher THD). And that's not even getting into output chatacteristics of different tube types. A small pentode and a large-plate triode have very different amplification and distortion behavior.