Was struggling with Mesa TC100 / Helix 4cm until last night. It required the use of two loops on the Helix: one set to Instrument level to Send to the input of the TC100 and one set to Line level to Return from the TC100 Effects Loop Send. Also, the 1/4 output on the Helix needed to be set to Line level to connect to the TC100 Effects Return. BINGO! Also, the little rotary switch on the back of the TC100 for selecting MIDI channel is one number off...to set the TC100 to MIDI channel 2, set the rotary switch to 1 (duh!!)