Has anyone experimented with the Electro-Harmonix English Muff'n and come to a favorite tube to swap, with a focus on bass specifically? It ships with two 12AY7s, so it's mostly a question of gain factor. 12AX7 increases gain while 12AU7 lowers it, etc. I don't plan to use it as a crazy distortion effect, but more as a boost. Please comment if you have a favorite tube replacement for punchy, clean low end. Also, how are the two tubes set up? If you were going to mix a 12AX7 and 12AU7 for example, what order would make the most sense?