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.
bsd512's post in Helix to Amplifi 150 was marked as the answer