REL speakon connection to amp vs speakers

definitely use the REL Speakon connectors from amp to sub...

I agree with the above advice and REL's recommendation.  You arguably get the amps flavor coming through with this method,  

I have also have pair of T/9x's and contrary to the above, I use the sub-out connections from my integrated tube amp to the RELs, RCA to RCA (low input not LFE .1)

I've done it both ways and prefer using RCA interconnects for stereo.

I also have a Yamaha A/V. My main speakers double as fronts, and the RELs are connected using the LFE input / RCA subwoofer cables so I use the RELs in dual mode (5.1 and true stereo) seamlessly.

The M33 has pre-out and sub-out connections that you can use for the RELs - if you use the pre-out, the REL crossover will handle more of the bass management so in your case either should work if you decided to go this route.

REL pushes the speakon connection partly because it's easier from a support perspective - everyone can use the speakon method.

If they didn't want you to use the pre or sub outs, why provide the extra RCA input?

I've spoken with REL support and both work, it's a matter of personal choice.

If you stay with the speakon connection, trim the cables!

PS. I know this wasn’t your question but it took me a while to figure this out so FWIW...

IMO integrating the subs with the mains works best with the subs on either side of the mains, phase set to 0.

Start with the crossover set roughly 5-10 hz below your speakers bass response or at the min for full range speakers. Gain maybe 10-15 clicks. Adjust from there. You should not hear the subs when they are set up correctly (or "see" them with your eyes closed) but miss them when you turn them off:)

Interesting approach I will have to try. My original question was connecting at the speaker or amp end.

But using the sub out does indeed mean I can use the m33 for signal processing (cross over frequency) but the reason for my question was actually I was thinking about upgrading the sub cable and I have 10 plus feet between the amp and the subs and the cost is way higher running the cable back to the amp.