Hi! I have a question about using tubes in my Bass 400+. Can I use it with 2 (or 4, 8, 10) tubes only? If so, how are they paired and which pair should I leave plugged in / which should I pull out (if this matters at all)? Is there any change in impedance with 2 tubes only? Do I need any special mod for this, I don't want to blow my amp. Thanks!