RSP is building really great sounding amps, really outstanding sounding amps built with an impressive craftmanship, no doubt.
AFAIK he's using Hammond transformers, which technically regarded definitely are of a higher quality than the Dagnall or Drake TX's of the old 70' Marshall amps, but...
... inside these slightly inferior transformers is buried the secret of the British sound, the British dirt, which you can't get with transformers of current production anymore. It's that special kind and mix of even order and odd order harmonics coming out additional of these OT's when pushed hard or into saturation - and it's that special kind of sag of the PT, when playing beyond a certain volume level and then when you strung the strings hard, followed by a interesting decay while you hold the note and let it decay, let it skip into overtones.
This kind of sound with the legendary British dirt in it and the behaviour of the amp is almost impossible to recreate by building amps, using transformers of nowadays - but with at least one exception... and to be honest it's slightly embarassing for me to point it out, because it's really not my intention with a current waiting time of almost 3 years, to pull the attention to one of my amp models.