so you aren't having an enjoyable listening experience beneath the 25% threshold? i would talk with Ray before you roll in a different tube that's not 12au7 compliant. i doubt changing to another 12au7 would in any meaningful way reduce this loudness. there are some amps i can listen comfortably and all day long with the volume cranked up loud and others i just can't without serious discomfort and ringing and for whatever reason i don't dig it at the considerably lower levels. at least for me i could never listen to my raptor at an enjoyable level because it hurt my ears. i believe i had some sort of sensitivity to that amp regardless of the volume level. what type of cables are you using? a good quality copper interconnect might make things a bit easier on the ears. it did for me, but not enough.
curious, are you noticing the same thing with your turntable or just with your digital source?
The input tube controls the amount of gain. I believe you can swap in various 9 pin tubes that are compatible. For example, if you liked to listen at even louder volumes, a 12ax7 could be subbed which is higher gain than the 12au7. 12At7 i believe is a lower gain and might work, there are also subs for the 12 series like the 5751 which is lower gain and would allow more volume control on the knob. Every one's listening preferences for volume is different, so he merely needs to find an input tube with lower gain and he should be all set.