Hey there. So I bought a pre amp/OD unit. It is the Blue Tube RT904. Rack mount, spiffy littly thing. What I like about is that it has line level I/O as well as instrument, so that i can stick in my effects loop and rack it. However I am not to happy with the results I am getting. My first thought is to change the pre-amp tube. 12AX7 I believe. Will be opening it up to see whats in side in the next hour or so. So my question is, what tubes do you all like for OD? A nice drive or distortion is what I am after and right now its a little buzzy. I like the JJ's in my 400+ but maybe there are better choices when pushing the tube into clipping. Well thanks all. Here is what the RT904 looks like.