I just spent the last several hours replacing tubes one at a time to determine why I was losing 90 percent volume after 10-20 minutes of play time on my 1+ year old Torii MKIV.

The problem occurred on both left and right but at different times.  Each tube replacement still resulted in volume loss until I replaced the KT66 output tubes.  Problem solved.  Right channel..... Then left.

At least one tube on both channels failed within a couple of days of each other?  What are the odds?  Is this an indication of a problem with my power (not conditioned, straight from the wall) or other common component?  I would have replaced all 4 as a matched quad anyway, but seemed odd to have simultaneous failures.

I don't play super loud. Moderate listening time in the evenings.  Any insight would be appreciated.

Andrew,
Over the course of about a year I experienced pretty much the same thing about 3 times with KT66 tubes. About every 4 months one or both of the channels would weaken or go silent.
I was convinced it was an amp issue.
I was on the verge of sending it back to Decware to diagnose.
As a last ditch effort, I decided to pop in a fresh set of EL34s instead of the usual KT66s.
That was about 10 months ago and the amp has run fine ever since without any issue.
I guess I was having bad luck - kept getting weak KT66s.