Varulf,if I'm understanding correctly,you have a "hybrid" amp,that is,it has a tube power or output section and a solid state preamp.If that is so,I have to say again,this is way over your head.You may very well need a new power transformer to support the additional tubes,quite possibly a different chassis,depending on how much room you have when you completely remove the solid state components,even if you can use the existing chassis you will have to cut holes for the new tube sockets.You will have to determine if the power tranny can supply enough current to add a couple more tubes,I would guess it wont.You are talking about building a high gain preamp to go into this out put.High gain circuits are a ***** to stabilize,even if we were to give you step by step,drawn out instructions,it is very likely you would not be able to trouble shoot it when you get instabilities from the layout causing squeeling and oscillations.I have been fixing and building tube amps since 1969 and I still cringe when someone brings me a high gain tube amp,there are many ocassions I wont even touch one.Look,like I said before,I would be happy to give you advice and to help you learn about tube circuits,but this isnt a project for your experience level.The fact that your first question involved cap values is disturbing because you arent even close to the point where you should be worrying about the cap values,you have to start by completely removing the solid state preamp,and again I mean no offense,but you likely cant identify what should be removed and what should be left intact.Okay,heres the answer to your first 2 questions,and we'll see where it goes from there.Your signal or coupling caps and tone caps should be 500 to 600 v's at least.Any preamp tube cathode bypass caps should be a minimum of 25 v's.Any plate bypass caps should be a minimum of double the voltage present at the plate of that particular stage.The values unless otherwise noted are usually in uf,if it were pf,it would say so.In fact most schems will have notes that say just that,as well as your resistors wattage.