Which tubes to replace Threshold SA/1 monos

Its a case of boredom. It will pass. You have amps that are benchmark products, even today.

I sold a pair of these last year... I regret it.
Sweet, huge sound.

Peter

Aesthetix Atlas Mono Blocks would have a more friendly input imp and match well with your Aesthetix Callisto pre amp
Best Johnnyr

You might want to take a look at Audiovalve Challanger lineup;I am driving soundlab m2's with excellant results using their Challanger 180 monoblocks;these amps will run el34,6550 and kt88's. They are class A and have balanced/rca inputs;excellant build quality and good worksmanship as well.

Thanks for the feedback. I put the First Sound back into my system over the weekend with great results and don't plan on making any chages for now.