You're gonna have to turn the 5150 down a lot to make it level with the Line 6. That head is actually 75w per side with 2 channels. You should try it out through both 412 cabs by itself so you can get the stereo image of the effects, too. Or if one of your 412s can be run in stereo mode, try that. It brings the most out of that amp.

But it will be nowhere near as loud as the 5150. Even the HD147 which is 300 watts (150wattsx2) won't quite do that because 120 tube watts is a lot louder than 150 solid state watts.

But if you can get your band to play at a lower volume that head will get quite a few cool tones that can really complement your playing.