nadernader1, the sound in your amp is "harsh" because you're probably mixing two amps/cabs together: the simulated amp/cab from Helix and your own tube amp/cab.
What I do with my Blackstar HT40 is the following: guitar into Helix -> pre fx (od, chorus, flanger, etc) -> send/return block -> cable from send1 to Blackstar's guitar input -> (here I "collect" the Blackstar's preamp sound) -> cable from Blackstar's send to Helix's return1 -> post fx (delays, reverbs, etc.) -> cable from Helix's 1/4" output to Blackstar's return.
That's the traditional 4 cable method. When you plug anything into the amp's return, the whole signal goes straight to Blackstar's power amp, bypassing its preamp (but because you already "collected" the preamp sound in the signal, that's exactly what you want). So you don't need (and probably don't want) other amps or cabs in your way along the Helix signal chain. The block that will take your "amp/cab" block space is the "send/return" block on Helix. And that will mean your Blackstar. Your Helix is there purely for effects.
Another thing I do is do the same route up to the point where I collect the Blackstar's preamp sound, but then I send the 1/4" stereo output to my monitors (they don't have xlr inputs, but you can use those if you have them) and connect the Helix via USB to the computer, so I can have a tone to record with (at lower volumes than the amp or even silently). In this specific case, I DO NEED a cab block, because I will not be using Blackstar's power amp, only its preamp. So after the send/return block, I add a cab or impulse response block to simulate my amp's cab and give life to my preamp's sound (or else it will sound feeble). Hence I don't connect anything to the amp's return (because I don't want any sound to come out of it in my room late at night hehe).
Another possibility in this scenario is connecting the Blackstar to the Helix through Blackstar's emulated output (into Helix's return), which in theory would eliminate the need for a cab block. I haven't tested this, though, and I think I like the flexibility of choosing the cab myself.
Does this all make sense? Hope it helps.
One important thing to keep in mind: when connecting the Helix's 1/4" output to the amp, go to the global settings and make sure it's set to "instrument". When connecting it to monitors, make sure it's set to "line". The same is valid for send/return: if you send your signal to the Blackstar's input, make sure it's set to "instrument". I think it's better to use one send1 block and one return2 block, because the Blackstar's send (or its emulated output) is a line signal, so it's better to have the helix's return set to "line". So by having send1 as instrument and return2 as line on Helix, you should have the most precise sound possible.