i used to own a YBA-1 many years ago. a couple of questions:
1) do you play with the amp on top of the cabinet?
2) how old are the tubes?
the reason i ask this is that i used to have the amp on the cabinet until i found out it shakes the tube elements loose, causing microphonic tubes, and actually in some cases breaking spot welds in the tube elements. this is EXPECIALLY true with bass amps. in many cases the microphonic tube will react at a particular frequency if the damaged tube element is of the right size to resonate at that frequency. with no input, and all gain controls up full, lightly tap on the tubes one by one with a screwdriver handle. any popping or crackling is a sign of a bad tube. any transmission of the sound of the tapping is a microphonic tube. when tapping on the tubes, be ready with the standby switch. i recently repaired an amp with a similar problem. when i got to the output tube that was bad, the problem was a broken spot weld on the grid. when i tapped the tube, there was a loud pop from the speaker, and the tube began glowing cherry red. always make sure the amp has a load. tube amps MUST NOT be run unloaded.
a couple of things help make your tubes last longer. play the amp on the floor or placed on something dense enough to minimize vibration. use ruggedized tubes if you can get them. mil surplus tubes will usually last a lot longer in environments where there is a lot of vibration. there are also some russian made tubes that are ruggedized for guitar and bass amp use and abuse. a good cleaning of all the controls and touch-up of solder connections in the amp is probably in order. you might also want to have the amp recapped as well. old electrolytics that are surely by now drying out will cause oscillation problems