First of all, you seriously need to reduce the size of that schematic image. It’s real difficult to read without special software.
That being said, FETs are typically biased so that their drain voltage is about half of the supply voltage. This ensures maximum voltage swing at both half waves. However, sometimes people deliberately choose not to follow this "rule of thumb" in order to introduce more asymmetry in clipping.
Properties of FETs vary a lot and even two devices from the same model batch can be quite different, therefore biasing with a constant resistance value may not work properly. Hence the use of trimmers, which is an easy way to dial in the right drain resistance (but not as good practice as using ordinary resistors).
One thing is for certain: You cannot just replace the tubes with FETs and expect the circuit to work identically. Sometimes it will not work at all.
What comes to biasing itself, try for starters…
http://www.colorado.edu/physics/phys3330/phys3330_fa05/pdfdocs/AN102FETbiasing.pdfhttp://www.qrp.pops.net/jfet-bias-2006.asphttp://ee.stlcc.info/132/fetbias.htmI bet you can find more with Google after that. Any decent book about analog electronics should have a chapter about this topic as well. I kindly recommend buying something like that for reference and studying if you truly are interested in circuit design. (Although some books just refer to aforementioned AN-102, which is a pretty comprehensive reference).
The presence and resonance controls will work as controls for bass -and treble response but they will be cutting instead of boosting. Placing them to feedback loop is mandatory for getting them to work like intended.
