One tube amp designed specifically for problematic low impedance loudspeakers is the Music Reference RM-200. It even has a 1 ohm tap! 100w/ch from a pair of KT-88 or 6550 tubes/ch.
Tubes with Infinity IRS Betas
I have always ran my mid/high panels on my Betas with a krell Fpb400cx. A friend let me borrow A pair of Primaluna Prolouge 6 Amps with Kit-120 tubes. They sound really nice but I am not sure if they are powerful enough for the Job. I have a line on a nice set of Vtl 300 deluxe mono blocks. I quess my question is what would I gain with the extra power. I listen at maybe moderate levels at best.