With other amps like the Naylor’s they were designed around 6L6’s by default, but I still much preferred EL34’s in them (some guys really liked KT88’s in it), this small Ronin amp I have came stock with 6L6’s (designed mainly for those and 6V6’s), but much preferred 6550’s, KT88’s and 6V6’s, the latest one I got has massive transformers (amp weighs almost 80 lbs) designed around KT88’s, but I much preferred EL34’s and 6L6’s. In all cases I heard the same relative differences I’m used to hearing in the different tube types
I’m sure the transformers are still a big part of what tubes sound good in an amp, but it seems to me IME more about just what tube complements the amp’s character like EL34’s just sounding good in Marshall’s, 6L6’s in most American voiced amps. Kind of like some people might look good wearing blue, others look good in red, etc. I think those amp guys saying you can get same results with eq setting are either not listening in a discerning enough way to tube swaps or don’t want guys go too crazy exploring the tube rabbit hole