First off, I have to say that I am very happy with the new WT-550 head. Great tone, headroom and punch, and warmth with a bit of growl if desired. You can dial in tons of deep tube-style bottom as needed. I opened up the amp to see what kind of tube was in there (I've been curious) , and I was surprised to find a Ruby 12AX7. It sounds so good that I'm not going to fool with it for now. However, I've read many posts here at TB from posters who have swapped out the preamp tube(s) for their various brands of bass amps; they talk about using 5751s, or various makes of 12AX7s, 12AU7s, 12AY7s, 12AT7s, etc. I was wondering if anyone out there has swapped out the tube for their Eden head, specifically, and if they have a favored choice which sounds better than the stock Eden tube?