Sorry, I had something really well written, and my phone decided to die on me, so atm, typing from my computer. To keep it brief, cutting the ground at the point he did was done to move the ground closer to where the other grounding points for the rest of the circuit tends to meet at (Where the Ceramic Caps are). Also those 4 caps near the tubes don't affects anything at all when it comes to the actual audio signal, those caps are there to regulate voltage and charge to the anode, which does affect the audio signal, just not directly like the red coupling caps on the PCB. The effect of putting those resistors and caps where the PCB connecting the transformer to the heater rails before they reach any of the tubes is that they're probably acting as dropping resistors, basically lowering voltage using resistors, and using the capacitors to possibly control the charge that goes into ground. All he did in cutting the ground there was to ground both rails at the same time instead of just one of them. The reason he may have lost that hum was that he reduced the voltage.
Also since you mentioned channel imbalance with the caps, those caps connect to the cathode pins on the preamp, and hold a charge, in a way, this is one method to bias a cathode, at least from what I understand, The capacitor's ability to store charges affects the biasing of the left and right triode cathodes, which that biased potential difference, in respect to the anode, may affect how the signal is receiving gain as well as how much each side is gaining. Basically, the capacitor's aren't correctly matched and may be working very slightly differently, and causing that imbalance. The caps I use are Elna Silmic II, 25V, 220uF, and they seem to be well balanced, so maybe try those?
Also, just a quick edit, but srsly, the Darkvoice's design is so weird, it has 200V 1000uF caps that receives the Bridge Rectifier's DC right? Then what do those caps do? They send that DC to smaller 350V (in my amp's case) 220uF caps that directly feed the anode. I feel like those 1000uF caps can be thrown out, and just replaced with 3 220uf caps and rebuild the circuit from there. The reason I think they might have done it with so many caps was to reduce ESR by using more capacitors, which I guess makes sense, but idk if it's really necessary in this circuit particularly, since it just adds more stuff to the circuit. Idk, I'd like some thoughts on this. I mean I left my design as is, but still tempted to try that out, since I'm almost 100% sure it's still function without blowing anything up.
