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.

This is how the bias meters look with the EL34s now.  

I know for the Taboo Mk IV when I had it I was told that as long as the tubes were generally 30 or above on the meters they were good.

James is right, how they sound is likely the best indicator of their health. In these instances I put a new pair in and compare the sound with the previous pair. It's easy to evaluate.

Well, hopefully nothing to worry about after the power uptick has ended.

It was caused by two days of cut voltage by Con Edison in NYC due to the heat wave.  At first, they had cut the voltage by 8% and by yesterday the voltage was cut so significantly that our lights were dim and our central AC stopped working.  It also caused a weird pulsing sound out of my speakers.  Eventually I disconnected my system and rode it out.  Weird stuff.