OK, read everything before writing ...
Have you tried swapping the L & R inputs? If the problem moves with the input connections, it's definitely before the Stratos. If not, there's more of a chance within the Stratos.
Unplug your amp, allow 5 minutes for capacitor discharge, then open the Stratos top cover. Carefully examine the cable connection to the binding posts on the left channel, wiggling them to make sure they are tight. If one or the other is not tight, you need to loosen the set screw, ensure complete wire insertion, and tighten that screw. The WBT posts use a Torx set screw that's an odd size, but I was able to align/tighten mine with an Allen hex key. (My Stratos arrived from its previous owner with a loose left channel, discovered before power-up.)