well, I put a relay in and could switch between different values I started with 510K and 340K. 510k was dull with the need of more sparkle on top. 340k felt just a touch bright. So I switched it and tried 360k and 480K both of these seemed fine with only a small difference. with the current speakers and tubes, I do think I like 360k best.. However, I am putting a microcontroller in to reconfigure the preamp tube, I'm planning to leave both values in. I will allow them to be switched in a configuration mode where the tubes aren't powered along with some preamp settings like 12/6 volt heaters. I didn't try the amp without feedback but I'm planning to try it..
I also plan to drop the value of Rd when I put KT88s in, which will slightly increase the voltage. I used an Edcor power supply with a little more voltage than the Hammond so I had to drop it a little more. I found the Edcor doesn't buzz audibly if you mount them on the rubber pads they sell and pay close attention to the resistance on the secondary. I had to add a 75-ohm resistor on each output of the rectifier to ensure the load was correct for a GZ34 rectifier with a 22uF load.
I am using good tubes (I have several sets) and capacitors. I like the Clarity caps better than Mundorf or Orange Drops, but both sound good (it's all a matter of opinion). I am surprised at how good orange drops sound for a few bucks.
The amp sounds really good. Thanks for everything you have taught me Skunkie! I would have never built a tube amp if it wasn't for you and a couple of other people you referenced on your channel (BlueGlow and Cascade Tubes) and I have an Electrical engineering degree but they don't teach this in school.