[xavriley]

This is just my two cents, but if you break it down into two components - frequency response and latency - then it's easier to imagine. With the latest modelling techniques (neural nets etc) it's possible for solid state amps to get very, very close to the frequency response of pretty much any tube amplifier in my experience. Expensive modelling amps and plugins are really nice. The second issue though is latency - when players say it "feels" different playing through a tube amp, my hypothesis is that they are noticing the latency (or lack thereof). Any kind of digital processing is going to impose some kind of delay compared with a tube amp which is a direct electrical signal. I'm aware that the delay will be on the order of milliseconds (sound travels roughly 1 foot per ms through air) and the effect could be replicated by moving the amp further away etc. It would also help to explain the change in sympathetic vibrations within the instrument. Shorter delays and high SPLs could have a non-trivial effect on the resonance of the guitar body for example.