04-11-12: DrummermitchellNot true. Only the later Sig MKIIa has the tube swap option. I ordered my Sig MKIIa in mid 2011 and it has the option.
I believe just the MKII as the IIA has that tube swap option installed already.
Drummermitchell (Threads | Answers | This Thread)
I spoke with Kevin and there's no circuit change, just a switch out of the signal path to deliver 6.3 or 12.6 volts depending on the tube type.