[R-390] Xtal oscillator module - Possible Mod

[Perry Sandeen]

TYo Bubba Dudes!.
First thanks to Rodger and all on the tuning details.  I never new it could be so complicated to tune up.  I guess I should hold onto my Western Electric 6AK5.
So here is my questions for those with more information than I.
1. The 6AK5 has a gain of 5,100 uMhos. It appears from the reflector observations that it is just marginal.

Could it be replaced by a cascade circuit of J309 Fets which have a gain of 9,000 (I don't know if that's uMho's or not) or something similar? 

What I see as an advantage is that gain would remain constant.
Or if one was so inclined could the 6AK5 be replaced with a higher gain sharp cut-off pentode?
2.  I've seen these DDS synthesizer boards (which I know almost nothing) on ebay.  Could something like this be used.  I do now that when one gets way, way down to the nitty gritty the frequency has minute frequency variations but I believe that they are so small that in our intended use, it would be irrelevant.
I noticed one small detail when reading the tube manual.  The gain went from 5,000 to 5,100 uMhos when the plate voltage went from 120 to 180V.  Might a low plate voltage be causing marginal operation?
Have the receivers with these problems had all their problem caps replaced as well as the Chuck Ripple note on changing the voltage rating of one small ceramic cap from 500 to 1Kv?

This is all just speculation on my part and wish that those who have more expertese would enlighten your humble servant.
Regards,
Perrier