That's not what I hear when I change cabs in my DT50. I plainly hear it changing the sound in the guitar speaker.
At first I turned off the HD cabs in the DT50 (as well as the HD models of course) and ran straight into the amp with the L6 link using just the preamps.
Then I tried running in with the amps and cabs of the Helix out of the L6 into the DT50.
Problem is...the way it should work is amp then cab. So pushing an HX cab into the DT50 power amp section is kinda backwards and always had a muffled tone.
My final solution after tweaking since Oct. of 2015 when I first got my Helix is: I use the preamps in Helix go out the L6 link using Topology IV only on the DT amp and use the HD cab inside the DT (whichever one fits your style)...after playing with that a bit and trying different cabs, I finally just used the HD cab model with a single celestion so that it "matches" the celestion in the DT combo amp itself.
That way I'm hearing exactly what the FOH is getting out of the back of the DT 50.
Having said all of that...I've kinda put the DT50 away now and I'm just going to a Bose speaker system to monitor myself.
The newest amp models over the past few months have gotten REAL good. And I'm using the PRS Archetype model for clean, crunch, and lead. It sounds really "real" and really warm. :)