Actually I think this is a great place to begin when describing the differences - which I'd say start with the 6V6 vs EL34 tubes. The circuits obviously create a difference, but a JTM being closer to Fender than a JMP will still have those stiffer qualities.What comes across really clearly to me in this comparison is how percussive the Marshall is. The Fender tends to sag more. Not necessarily a blanket statement to be made about Marshall vs Fender.
Try standalone speakers or headphones? There's a whole industry of demos on youtube that seem to thrive despite your assessment.I don't give any credibility to this type of test.