These videos compare an 80WPC P/P KT-88 Melton with no feedback used in the circuitry vs a Yamaha RZ-Z9 Receiver in "Pure Direct" mode. From my understanding, no feedback leads to a "softer" treble, and that is certainly obvious here.
Every amp is just a bit different, and there would be more variation in tube amps than solid state, so take it for what it’s worth.
All levels are within a couple tenths of a dB using pink noise. While the difference is instantly obvious, you need a good set of monitors or headphones to appreciate all the nuances.
These are recorded on a Nikon D750 DSLR and no eq or room correction, either electronic or physical is used.
Pull them up in two browsers simultaneously and go back and forth for the best comparison. This is something you would never be able to do at home or in a shop using the actual equipment and shows just one of the values of YouTube.