Thanks so much for your response! The known good tank has no markings on it other than input and output, unfortunately. The resistance was around 180 ohms for both input and output. Thank you for your suggestions on replacement tanks. I've read elsewhere that the new tanks sound different. I've seen a number of "working" used Gibbs and Accutronics tanks on ebay. I wonder if that might be an option if I can find one with appropriate impedances.
I contacted the original seller and asked if he has the original tank. I would look into getting that one repaired if he has it.
I am impressed with the sounds I'm getting with these amps. I've been running it into a 4 ohm load because I don't have the bottom here in Brooklyn. I think it's looking for 8 ohms. Will I cause a problem long term running it into 4 ohms, do you think?