Firstly let me say I love speakons. I have connected hundreds and as has been said check the screw connections. you need to use a good screwdriver to get a tight grip on the cable. I seem to remember that it is a 1 point posidrive (not philips)..Don't even think about using 4mm2 cable as is too bag for the plastic speakons and if a far heavier gauge than you need. 2.5 mm or even 1.5mm cab is fine.
Tighten the screws as hard as you canand then pull the cable as hard as you can to check that you have a good grip. It is never a good idea to daisy chain cabinets, use two separate cables as most good amps have two speakons.
As for speakon wiring, 1+ to 1+ and 1- to 1-1 on both 2 pin and 4 pin connectors. The 2+ and 2- are generally used for subs in a bi-amplified system. There are other proprietry wirings that rarely come up.
Going back to the system as a whole, in the situation you were in you should have just bypassed the Helix and gone straight into the amp.