I think I've read something about the brain being able to use harmonics of the frequencies in question to "reconstruct" what's lacking. IIRC this is how Waves MaxBass works. This could be an added effect of your tube gear, which will be generating more harmonic content.
Another thought I had is how filters differ from an organic lack of response. Filtering wide band audio introduces artifacts. Those can be heard on AM every day in the US in the stations using brick wall type 5Khz filters.
A wideband signal run through something with low response is different!
And the cutoff matters.
No matter what the gear, a 5khz cutoff is crappy. 8khz can be good, and 10 can be hard for many to tell given good gear and quality source audio.
Once, I was able to listen to a great jazz recording on a older tube stereo. 50's era gear.
Playing vinyl on it was a terrible experience for both the listener and the vinyl. A recording done on tape was kind of amazing! The device was mone, pretty sure was a Zenith, and it had a cool baffle system on the speaker. One "tuned it for the room" by pulling a knob in and out. That shaped the low and mid range.
Was better than I would expect for that overall bandwidth.
