Output impedance is a real issue with tube amps, even those with feedback, because the feedback is not always fully operative due to load reactance. The definitive solution is to use a great many output tubes.
Charlie, I pulled the feedback from the EL34 version of my V8 some three years ago. Its got a bigger soundstage and is more vivid in the midrange, but I can't say as I like it any better overall. I am having some luck, believe it or not, with 6550 beam tubes using *local* feedback. This converts the horizontal plate curves to triode-like diagonal curves. These 6550 amps don't use any global feedback. They were used to drive the Avalon Opus at the recent HE2003.
But I have not given up on feedback. I find it a facinating challenge. The reason I think it can work is because it is used throughout the human body. Even the ear uses it. It is what protects the inner ear from overload. The touch of a finger on a piano key is a servo operation through and through. There is even compensation by integration. So I believe feedback can work. It's just difficult.