ARC LS7 preamp channels drop out intermittently

Hello all!

I have an Audio Research LS7 preamp that, AFAIK, still has its original tube set. Recently, its developed an issue where one of the channels drops out during listening, or fails to operate after powering on. The issue is not isolated to one specific channel...it can happen to either left or right, but oddly enough, never to both at the same time. I can usually correct the issue by shutting it down, waiting a few minutes and turning it back on, but recently I’ve needed to do this 2-3 times to get the affected channel working again. I’ve pretty much isolated it to my pre-amp; both my channels are driven by ARC D240 MKII solid-state amps running in bridged/monoblock configuration & all my connections are good. Speakers are KEF Blade Two’s that I bought new about 5 months ago. Could this be a sign that my output stage's tube is failing?  As this is the first tubed gear I’ve owned, I'm not experienced in troubleshooting or knowing when tubes start to go bad.  

Appreciate any thoughts or suggestions...Hope you’re all doing well!

Arvin