I purchased an inexpensive 15W Tube amp for my 12YO Grandson It is of Chinese manufacture w/ 2 ea. 6V6 power tubes is a 12AX7 in the phase inverter socket It has a solid state preamp. Does it matter what brand / value of 12A** tube you use in the phase inverter? Will a Mullard 12AX7 give a better tone the the stock Chinese branded 12AX7? Will a different value, like a 12AY7, 12AU7, 12AT7 be better in the phase inverter position work better Does any of it matter?