How to adjust gain in FET based circuits
To change JFET stage's gain, change the unbypassed source resistance.* ... You can change gain easily by inserting unbypassed source resistance, and probably should do it that way instead of swapping JFETs at random. The J201 is a high gain device, so it's easy to turn the gain down.
Our basic rule of thumb has been that if a bit less gain is needed, swap out the J201 for an MPF102 or 2N5457. A side effect is that the overall tone will become brighter.
However, those three types were usually only ones we'd try while developing FET-based amp emulators.
We didn't choose those types according to datasheet specs; they're simply three of the more common FETs in the realm of DIY-fx. It's not very scientific to be "easter-egging," but doing so would sometimes produce pleasing results.
Text taken from this thread.
* paraphrased
