Just replace the input 1 Meg resistor witn another volume pot, 100K should be fine for this purpose. THis will allow you to attenuate the input signal so that it won't go into distoertion in the first stage. Then use the main volume pot to set the actual output level.
This way, there are no major circuit changes, to a proven design.
The SS FWB rectification is better for your application, as the usual tube rectifier PSU "sag" would be deleterious for a keyboard / organ amp.
It would be good to use as much capacitance in the PSU as you want, the built-up stiored electrical energy in the bigger cap(s) will handle the volume peaks a lot better. 33uF is quitesmall, especially for the first "A" cap, which supplies the B+ to the OPT and EL84s, and can cause unwanted B+ sag. . I'd use at least 100 uF here, and 220, 330 or 470 uF would be fine. Use 450V rated caps, 300V or 350V would be cutting it close, especially before the tubes are warmed up and drawing current.
The two other 33 uF PSU filter caps are OK, and a J/J 32 + 32 uF / 500V dual section cap coule be used for these, or two separate caps, your choice. These both could also be increased to 100 uF / 450V.
One caveat - I ran into this particular issue on a build about three years ago, and found that the Hammond 261M6 power transformer throws a very strong 60Hz magnetic (H) field, and any open frame SE or PP OPT can, and will, pick up the 60Hz induced field up to nearly two feet away!
Note: this was 60Hz AC hum, and not 120Hz PSU ripple (which is the usual "demon" in DIY tube amps)
I used 125ESEs in my testing, and orienting them orthoganally at 90 degrees only reduced, but did not eliminate, the magnetic coupling. THis shows up as an "incurable" hum problem, even if the power tubes are pulled from their sockets.
If you use a steel chassis, and mount the OPT inside the chassis, and the 261M6 PT outside, or on top, of the chassis, then the steel should provide sufficient shielding, though a steel bottom plate may be required for total enclosure. Aluminum does not stop or contain H-Field (magnetic) radiation / coupling.
See my next posting for a reprint of this testing
/ed B