> Secondly I think it's important to note the tube amp and the guitar are seperable, and I don't think that their connection is particularly magical. I can reamp a sound from my synthesizer (or maybe a keytar?) into a guitar chain, and if I manipulate the mic and other controls in the same way I might manipulate the pickup, I can also get all manner of interesting feedback effects.
The story is not quite so simple. Your synthesizer is going to have a buffered output so it wont have the complex impedance loading interactions with the amplifier as the guitar pickup.
This is actually critical to how early distortion effects such as the classic Fuzzface work and imo is essential for the kind of complex timbres you can produce with a guitar + tube amp.
In fact you can take an electric guitar, put a buffer pedal in the chain between your fuzz pedal and amp and completely destroy the ability to produce wild feedback and distortion.