ProAc ER1 sub connections to Aktiv Amps

Sorry, I've never encountered a Neutric connection. Any line out RCA from any amp is the same as one of the two RCA line outs from your preamp. That means it is a full spectrum line level signal.

Based on your opening statement I assumed you had the sub working fine. Where in the world does this Neutric connection that needs the HF speaker out information go to? The speaker out would mean an amplified signal meant to drive a tweeter. At least that's how I'm reading your post.

Neutrik, is the correct spelling, my mistake.

The amp section in the ProAc is the same as the REL Storm III. The Neutrik connection is 3 wires connected to the right + - and left + of an amp, on the other end is the Neutrik Speakon Interconnect that goes into the sub. This interconnect is supplied with the sub.

This allows the sub to be optimised for 2 channel stereo as it recieves the full signal.
This connection is preferred for 2 channel setup according to the manual.

As I have 3 amps, I dont know which amp to use the connection on.

I dont know if it is redundant as I have the full signal going to the LF via interconnects, but the HF knob does nothing. I would like to try the HF signal to listen for any improvements or if there is actually any difference.

I dont know if I can damage the sub or the amps by using this connection, I dont know if the fact that the amps are Aktiv makes a difference as they dont all send a full signal.
Should I connect to all three of them??

This is an interesting question. Sorry, but I don't have a clue. My best guess would be selling your existing sub and buying one of the Linn subs would be the logical solution.

I connected to the Bass amp with the Neutrik connection.
Using the HF connect made a noticable difference, the sub blends really well, highly recommend this sub, it is fast and deep. And adds a nice blend to music, Patricia Barber's Modern Cool, was cool.

I'm glad you have succeeded. I've learned something as well.