What made you change to a 6SN7 preamp?

I think you were reinforcing the point I was trying to make that it's not all about the tube? 

That is true. But if you work with tubes over time, you get a feel for what they do. More to the point though that in order to really compare what the tubes do, you have to make sure all the other variables (circuit topology, part quality, grounding technique, power supply design and so on) are put to bed.

We see this going on in the SET world all the time- SET fans say they've compared SET to Push-pull, but when you dig into it you find out that the variables I mentioned above were uncontrolled in the comparison. This makes comparisons invalid; all you can conclude at that point is you might prefer one over the other without knowing why that is if you're being truthful with yourself.

I was immediately struck by how much quieter it was, how much detail it had, and how it presented as good or better of a soundstage and had very precise imaging. 

It would be lovely if we could attribute the characteristics above to a particular tube; we could all just use that tube and then have state of the art. To address those characteristics has everything to do with operating points, bandwidth, parts quality, grounding, layout, power supply design and so on. The advantage of the 6DJ8/6922 is its linearity and bandwidth (it was used for instrumentation for example) so if you have a non-microphonic tube you'll do well. Since microphonics was not a concern in the applications for which this tube was designed (like television tuners) finding non-microphonic examples is a bit of a task.

In building preamp amplifiers, the 6SN7 based tube in general is a warmer sound, more relaxed presentation, more dimensional soundstage with real mid-range beauty.  That being said, the 6DJ8/6922 is generally more dynamic, faster, better clarity, and more slam.  It is a more upfront presentation.

6DJ8s were not designed for audio and have trouble with microphonics. I find it far easier to find a low microphonic 6SN7. Microphonics tend to add an iridescence to the sound that is distortion, so despite the nice linearity of the 6DJ8 I learned to stay away from them.