OMG bro, I’m having same issue with my Helix floor running 4CM into my Marshall DSL100HR and 4x12 cab. I use fx block(s) ALONE on a preset,and get mad tone suck. Like you said this shouldn’t be. Why would the processor alter the tone. It’s not being totally bypassed I assume. I tried also the levels and such on the send/return block(s) and still same results. This amp kicks lollipop has tons of aggression and balls running guitar straight, but through Helix I get tone suck,and all that is lost. Thought I might be doing something wrong. Maybe I am. But for you to have same issues I think not. If you figure out a solution please inform me some way. I will do the same.