Ah, those figures seem more reasonable - 30pF seemed a bit low.
Well, I'm not sure what you should be on the lookout for - as you say, it's a completely different animal to having a 3uF in line with the S2.
What I would suggest, and this isn't too difficult given that you'll be using an adjustable aircap, is this:
1) Try the initial setup, ie with the 300pF
2) Add a 150k in parallel with the 150k to give 75k. Double the cap to 600 pF. See if you can tell any difference. I assume your preamp is just as happy driving 75k as 150k and the cables you are using don't have too much capacitance.
This assumes your aircap will got 600pF, which is getting quite big. If you don't mind changing the grid resistor to a higher value, you can try smaller values of capacitance. If R is the resistance in kilohms and C is the capacitance in pF, just be sure to keep the product, RC, at 45000. Eg try 150pF and 300k, 96pf and 470k etc.
I myself am building a 2 stage amp to drive some ribbon tweeters. The input tube will be E810F triode connected, which has an enormous Miller capacitance, so instead of an RC at the input, I decided to try a stepdown transformer which I wound myself. The filter is formed using a resistor in series with the primary inductance, and the stepdown really reduces the Miller cap of the E810F. The tx also enables direct coupling to the output stage (-ve supply) which will have an air-core tx, giving an additional rolloff at 600Hz to protect the ribbons. No caps at all and I do away with the transformer on the ribbons as well...
The toroids are less than 2" dia. and made of a rather special cobalt amorphous alloy. Wound with Chimera labs single crystal copper (which is probably about a million crystals after the winding process).
Anyway, as I'm some way off from getting the rest of the project sorted, I could send you the transformers to try out for a few weeks... preamp would see a load of 20k and up. I'd be *very* interested to hear how they compare to the aircap in your setup. Lemme know.