This might be a bit late but I also had a pair of Definitive Mythos STS speakers in Toronto and the amps died. Apparently fairly common. I took them to Frank at Speaker Shop in Scarborough. He said he wouldn’t even try fixing the amps given Definitive’s lack of support for them and the poor documentation (he said the guy who designed them used to be pretty responsive but he died and there are no good schematics). However, he rigged them up passively with crossovers for me so the woofers are just driven by my main amp. Don’t remember exactly what I paid but it was around C$400.
They sound great — as good as before IMO. I know people sneer at those speakers because of the design but I have always thought they sounded excellent, even compared to other speakers I’ve had that would get more respect. Of course they are not hooked up to the line level RCA jacks anymore; they are just full range speakers. I have a separate sub with them right now, but they’d probably be fine if I just told the amp I had no sub and had it send all the bass and LFE to the L and R speakers.
Just an option for you to consider. Worked great for me when I had the same problem you seem to have.