I've done the swap of 12AT7 in place of 12AU7 and with the tube gear I had the difference was pronounced. I definitely had higher gain and didn't have to turn the volume knob up on my preamp as I did when I had the 12AU7 tubes. The sound also was a little more forward but I had a little more detail. This was in my system though. With the gear you have you may hear something entirely different. Since I like to turn it up I preferred the 12AU7 tubes in my situation.
