When I had my Amplifi 150, as an experiment (because someone on here asked), I used the 1/4" inch Helix outs with an adapter for stereo in to the Amplifi 150's aux in. It sounded plenty loud and clear to me. Maybe experiment with the Helix line level/instrument levels on your 1/4" outs. I was using a pair of Yamaha HS8's as my regular setup which sound considerably better of course, but the Amplifi was certainly acceptable, and plenty loud - easily capable of more than I'm comfortable being near for very long for practice.
I've since given my Amplifi 150 to my son along with my Epiphone Les Paul, so I don't have it anymore.
I think others have said that using the Helix headphone out also works well into the Amplifi aux in, so maybe try that if you're not having success otherwise.