I think there are only 2 ways to adjust the overall volume of a guitar path without effecting tone: channel volume and output block volume. I always use channel volume because all you have to do is hit the amp button and the channel volume is instantly accessible. To get to the output volume you might have to push the joystick several times to get to the output block -And risk accidentally turning the joystick...
I haven't used a K12, but it looks like there are several factors that could boost the volume. Most noticeable, If you use the 1/4"/XLR input and have the switch set to microphone, but Helix output is set to line level, it's going to be a hot signal going in and the K12 is going to really boost it, making it crazy loud and possibly distorting it.
I have a L2M and really like it. It's my stage monitor for my guitar and vocals. The loudest I've had it on stage is about 1/2 volume. At 3/4 volume, it could blow your hair back!