Beefing up bass amp with 2 6AQ5 ...

[Printer2]

The output power of an amp is based on the PT capabilities and the OT primary impedance. In the amp as it is, these parameters are set and there is nothing you can do about it.

The only way you might get a little more power is if the amp is cathode-biased and you change it to fixed-bias. This will make the amp more dynamic BUT may not increase useful output, since full-load is still the same.

You would be more successful changing the speaker complement and cabinet positions. Use two cabinets separated by a space and let psychoacoustics work for you.

Higher voltage with SS diodes and a tube that can use it into the impedance. Or if the OY has multiple taps for the output you can go up a step in impedance without SS diodes, or if the transformers can take it, with.

 

[AquaTarkus]

...had some looks and thoughts. I think I follow a gradual approach.
1. Change to solid state rectifier. This will increase the B+ to 375 volt, and save on heater power.

2.a. If not change to a pair of EL84s - What output to expect with B+ =375? (JonSnell suggestion)
2.b. If not change to a pair of 6L6s, while watching temperature of power transformer.

You should be able to get 15W fairly clean from a pair of EL84s that way, possibly up to 20W with a little distortion. If this is a bass amp it doesn't need to have hifi distortion levels.
As has already been said, 6L6s are too big for relatively little benefit. Same goes for EL34s and KT77s. The fact that your transformers appear oversized is actually good for bass response.