Lower power on left channel

It's almost certainly a tube or tube going bad you have some good advice above.

I had that problem a few weeks ago and it was the right channel with my pre amp that was causing the problem so I replaced it and all is good as new.

Good luck, OP!

Seasons greetings to you Bigred,

just a quick fyi that Prima Luna amps are equipped with a “BTI”  (bad tube indicator).  It’s an LED that will glow red located in front of each power tube.  If no red LED glow your tubes are good to go. 
 

Enjoy the music. 

I’d work back from the speakers. Swap the speaker cables at the back of the amp. If the trouble follows the swap, you’ve ruled out your speakers. Swap the cables back.

Next swap the cables from the preamp. If the trouble moves, it’s the preamp or source. Swap the cables back.

Swap the cables from your source. (CD, DAC, etc). If the trouble moves, it’s your source. If it doesn’t, you’ve ruled out everything else and it’s your amp. Swap the cables back.

Now you can start swapping tubes around with some confidence to help pinpoint things. If tube swapping doesn’t make any difference then you have another issue with your amp and unless you’re well versed, you might need to seek professional assistance.

Almost my entire career was in IT, both hardware and software. The trick to this kind of troubleshooting is no trick at all. It’s being methodical and thinking the problem through. Best of luck!

Happy listening.