I have an SVT3-PRO which has 4 Groove Tube 12AX7's and 1 Groove Tube 12AU7. I don't want to change the tone of my amp any, or nessasarily give it more gain or bite or anything. I just want to know which is the best replacement tube to buy. I know Groove Tubes are known for lasting a long time, but I've also heard that some of the smaller names are better for durability etc.. The amp isn't that old so it will be quite a while before I have to replace, but this is for future reference. Also, is chaning preamp tubes something I can do on my own the first time? And will I have to rebias if I change brands? Would it be a good idea to buy spare tubes to have on the road just in case? I know, a bunch of questions but any help will be appreciated.