+ 1 to what Booyah said. I cannot imagine the ZLX112P being not loud enough. There must be something else going wrong here.
As for the DXR15, it has been checked against the 10 and the 12 by several users and the overall estimation is that the 10 is the leader of the gang.
As I said, the ZLX112P would probably work perfect on stage but we are much louder in rehearsal. I don't know why? Loud drummer perhaps. The sound guys always goes: "Turn down your amps!" when we're soundchecking... I once let a sound guy hear our rehearsal volume and he gets that strange look on his face... Haha.
We are all using earplugs btw.
Regarding DXR10, 12 and 15 i've read some great things about them here on the forum. One that said: DXR10 is the winner, but if you like some flapping pants bottom end, go for the DXR15.
We have been doing this for over 20 years now and I think it take some time to get used to FRFR, and that the other guitarist, using his Randall V2 with Mesa oversized cab maybe should turn down a little or go FRFR as well?