Are the tubes on the SVT 3-PRO easy to change when they become old? How long should I expect them to last? (I do an average amount of playing). Are they soldered into the amp or do they snap out with ease? Also, let me get this straight. It uses three 12AX7s in the preamp, and a 12AX7 and a 12AU7 in the driver section. I know the power amp is MOSFET. Whoever has experience with this amp, please give me your thoughts about it. How well will it push a pair of SVT410HE's? Thanks in advance, DAN