Which REL subs for ProAc One SCs?

Jim, yes REL provides the Neutrik cable, although you can also buy after-market upgrades too. I had one made by somebody here on Audiogon some years ago, now I see that Kimber makes one specifically for REL subs, I think Signal Cable has one too. Try the standard one first, and follow the tuning procedure in the REL manual. Setting the crossover LOW (perhaps counter-intuitive with such small monitor) works better 90% of the time, but your room and ears and music will be the guide.

If you are concerned about overdriving the main speakers, you should go with a fully active crossover/subwoofer system that actually cuts off the main speakers below the crossover point. With such a setup, the best sound will most likely be with the crossover set at a quite low frequency, as others have mentioned above. I almost always find that for music, a low crossover point sounds better.

I haven't listened to that many subwoofers, but, of those I heard, I liked the Martin Logan subwoofers that I heard coupled to quite small Totem monitors.

While the REL does not cut off the main speakers in the low frequency, I have been impressed with how easily REL subwoofers can be integrated into a system.

River251, I recall you being located in Phx. Check with AZ Hi Fi re the Gallo subs. More flexible in setup than Rel and deliver a sound that isn't designed as "bottom up".