Here's my novice attempt at helping:
1) The 1.5K resistor value is important because changing it will also effect the voltage drops downstream to the AF/PI plates and the tuner voltages as well.
2) Dave recommended a screen voltage of about 350v to be a better match to the OT primaries. You know that everything downstream of the 434v B+ is drawing 30ma because of the 45 volt drop across the 1.5K resistor R126. 45/1500 = 30ma. You want to move the screen tap further down in the 3700Ω (2.2K+1.5K) network. To get the 84v drop you need for 350v on the screens, you need a 2.8K resistor to replace R126 and then a 900Ω to replace R123 which will keep the total at 3700Ω as before. This should drop the screen voltage to about 350v and still maintain the original design voltages downstream from that. For simplicity you could probably use common 2.7K for R126 and a 1K for R123. R126 will be dissipating 2.5watts and R123 .9watts, so (over)size accordingly. Keep in mind that the experts have said that using more resistance to drop the screen voltage will result in poor screen voltage regulation which means increased distortion, but at least you might avoid burning up more tubes.
3) To get your idle current in the ballpark, I believe you need to be more negative on the grids, so adjust resistors R104 and R125 to drop the voltage, but again keep their sums the same to avoid changing other parameters. Do you have a way of measuring your idle current on the 6L6's? I know it's common to use an adapter on guitar amps that sandwiches between the tube and socket which allows you to insert an ammeter on pin 8 to measure cathode current. Someone else will have to chime in on where his idle current should be.
Someone please speak up if I'm giving bad advice here. I honestly would not be going this way, but would be looking into building an EFB circuit and fixed bias supply as was Dave's minimum recommendation. With that, you can know that your 6L6's are operating exactly in the best possible configuration, producing the most power output, and will live a long happy life
. From there you can choose to address the other issues previously mentioned if you feel like the sound can be improved upon to your ears.
BTW, you mentioned changing to tube rectification. I see this is using a voltage doubler circuit which means you cannot replace the two silicon diodes with a standard full wave rectifier tube since the tube plates share a common cathode. You would have to use 2 separate tubes to make the conversion - either some uncommon tube single diodes or a pair of standard full wave tubes using both halves in parallel.