Trying Tubes in a Crate V33-212

I like to tinker. So it should be no suprise that now that I have a tube amp I want to do a little testing to see what it is capable of.  I emailed Crate to see about the best way to get into the amp and they provided me with the Final Assembly Schematics for the amp, which showed that removing 2 screws allowed the amp assembly to pivot down.

So what you see here is the transformer, the 4 EL84 tubes and the 3 12AX7 tubes.  I borrowed a 12AU7 tube from a friend to see what that would do inserted into the V3 position (the leftmost tube in the front row of this picture). Being that the 12AU7 has a substationally lower gain than a 12AX7 it cleared up the clean channel which when it was on 0 volume had a slight low hiss, but since now the power tubes were getting a lot less power from the inverter tube, it was much quieter. I did have to turn the amp up much louder to get the same volume and it started to break up the clean channel at a lower volume then before. But there was a bit of fullness that I seemed to lose. Not sure I am ready to make this change. I need to test some more first. My friend also recommended rather than a 12AU7 a 12AT7 which has a gain level somewhere in between the U and the X. All I know is that either way the amp sounds really good, I'm just not sure which way sounds better yet.