Likely the left power tube is getting weak. This isn't necessarily a problem, yet, but IMO you should try replacing the power tubes with a new (but broken in) pair and compare the sound. If the old ones still sound good you are OK, for now. If not, thank them for their service and dispose of them. If you're going to replace, replace them both.

If you don't have at least one complete set of replacement tubes you should get it for two reasons. One is the best way to tell if a tube needs replacing is to compare it to a known good one. The other is tubes can die at any time so you want to have replacements handy to avoid downtime.

Quite a few of my matched output tubes read 1 notch above or below one another on both Rachael and ZenUFO amps. In fact I just bought a matched quad of Decware 6p15p's with 2 of the tubes reading 30 the other 2 at 29.
In past my amps have played/sounded fine with this status and I think Steve mentions that at least in the UFO84 they balance each other out internally.
Because I rotate amps and tubes often I have yet to wear out a set of power tubes.