1)The balanced PI is just a tube that is tested and selected because the two triodes are matched much like 2 power tubes are matched so as to drive the two sides of the push pull output evenly.It wont hurt anything if you put it in another position,I personally dont see a need for it in the PI position,gutar amps sound good with a certain amount of "non-linearity" anyway,so why not in the PI.2) Preamp tubes dont have to be the same brand or in fact the same type,if you like a 5751,12AY7,12AT7 etc. in one position instead of the 12AX7,it is fine,it is all a matter of taste.3)Power tubes have to be closely matched,if they are matched within 5ma's or so they are okay,but dont try to change just one of a pair unless you know it will draw close to the same current as the other. 4)You may see one manufacturer recomend xxx amount of volts for one EL34 and another recomend yyy amount of volts but these are not hard and fast numbers,any EL34 you like the sound of will work in any amp designed for the EL34.Hope this helps.
