Cool. Not a hard thing to find these days at all though, so it seems likely that there's a at least a bit more in play here, no?
When I was down this rabbit hole a decade or so ago I built a tube preamp just to see how flat I could make it. And by flat I mean measured pretty danged close to dead flat from 20-20KHz:
Predictably, it was not all that much fun to play bass guitar through without some flavor being introduced elsewhere in the signal chain. But since it had no knobs there was no possible double meaning for "flat", at least!