About the capacitors, I'm using the exact same caps as you are, JJ electolytics and motor run oil caps. I'm not so fond of parts rolling and generally try to find improvement on the algorithmic front (the circuits) first. But I may try some different brands when there is nothing else left to tune.
About the driver stage, yes, up until now I've always had the VR tube in place. If you leave out the shunt regulator on the driver (but keep the CCS) and you remove the capacitor between the centertap of the output transformer and the virtual cathode, the operational mode of the driver shifts from PP to differential (forced balance). Lynn once wrote that he prefers to "let the tubes find their own balance", but it's easy enough to try both to see what you like. The shunting cap (WE cap) is IMO one of the most interesting parts of the design, although I have not been able to get it to work as intended (reducing 3th and 5th harmonic) yet.
So many things to try and so little time :-)