What about a tube amp that oscillates, motorboats and has runaway bias when inputs and outputs are not connected? Is it reasonable to design a tube amp that is only stable as long as it is not powered without the source and load connected?
Not really, no. I wouldn't sell an amp that wasn't stable even if the user trips over the speaker leads.
Instability is almost always linked to caps and phase shift though That and cheap transformers.
Instability is almost always linked to caps and phase shift though That and cheap transformers.
But do some designers do it anyway, and just tell customers to always keep the source and load connected? How common is it?
Probably more common than we think. Sony doesn't do this (They make their crap to a price point, but it's not garbage). McIntosh probably doesn't do this. I certainly don't. I'm sure there are many people selling garbage as "The best amp since the Audion!".
Are you overlooking the fact that the series cap greatly reduces the power dissipated in the resistor?