Funny, I was just tweaking mine yesterday!
I have a late 50's Bell & Howell filmosound projector amp, 2 Cathode biased big bottle 6L6's with a 6SL7...and an awesome 16ohm alnico speaker, In a box that looks like an old radio! it's only a power amp, so you had to drive it with a preamp.
driving it with the line out of my hotplate sounded great!
I used to use the speaker with my 73 50w Marshall. Then a few years ago I stuffed a 12ax7 wired in parallel in it to use as a stand alone amp.
It used to oscillate when cranked and was a tad noisy which I chalked up to the samll size and induced hum. I had a 12AU7 in it, to cut back noise but it sounded a little weak.
So yesterday I was bored and started tinkering with it....turns out I had positive feedback going on!
So I removed the feedback, put in a 12AX7, and damn if it doesn't sound awesome!