These amps utilize a pretty standard "Fender style" phase inverter and power amp. Decreasing the resistance to the screens increases the screen voltage slightly. This reduces the impedance of the tubes and causes them to run a little hotter. This has the effect of attenuating the high frequency roll off (warm sounding) that 6L6s are known for. This is just one of the reasons that fender amps often sound so ice picky. I am convinced that many amp techs (Leo Fender included) lost the high end of their hearing which caused them to design shrill sounding amplifiers. I have substituted a spectrum of values for the screen resistor in many Fender and home made amps. If anything I would recommend going to a higher value; maybe as high as 5k for the screens, this seems to yield a more grainy sounding texture. Higher value screen resistors also tend to protect the output transformer to some extent in case of tube failure. As always; the proof is in what you like to hear and how you want to sound , so experimentation will yield the "correct" configuration for the individual player. I would also recommend trying substituting good quality 6V6 power tubes along with cathode biasing. I love the sound of 6V6 power tubes and cathode biasing in Fender style amps. I built my OTS with a Fixed/Cathode bias selector switch and I seem to leave it in the cathode bias position. I'm in the process of adding spring reverb; which I seem to be addicted to...
It would be fun to add the mods that you recommend sequentially and analyze them on the scope to see what results.

Thanks for the info JohnnyMac.