Use a CCS instead of a resistor, or perhaps an unbypassed R for the cathodes. I suspect the R will give better overload performance than the CCS.

I'd rather build a low gain pre like Guinevere with 12B4's and roll a bit of gain into the adjustable volume control/buffer stage. Or take 12B4's and keep the 5687 gain of Guinevere. there is something to be said of two stage amps and linestages with gain. Work together on system requirements instead of building the whole thing to a specific standard of sensitivity( which will set limits on tube choice somewhere in the chain...).
regards,
Dogulas

Praise the Lord, and pass the 813's!