One other thing that comes to mind, is that SP units have a current programming resistor in the head, so you cant overpower a small tube with the big psu, Since that and a few other wierd interlock things occur on 261s, hit the FAQ.
My experience with 261s is the controls and current limiter are solid, but the meter driver opamps are easily blown, mine has a misreading light meter right now, blown opamp. So before you assume your getting 12 amps of plasma,, check for a shorted passbank transistor, or a blown current sensing opamp.
Steve