Anyone know how to set up Rythmik subwoofers on an ARCAM AVE550 receiver?

Call Rythmik and talk to designer/owner Brian Ding. If you want to connect the subs to the speaker output terminals (binding posts) on the receiver, when you order the subs ask for the PEQ or PEQ3 plate amp. Both include binding posts for high-level (speaker) connections.

I have talked to both Brian Ding and Enrico.  They both told me to use the LFE hook up with my ARCAM and let the ARCAM DIRAC room correction handle it.  They also told me to set my ARCAM speakers as small.  Seems to me if I set them as small it might eliminate the two woofers in my towers and have them act as monitors.  This to me would defeat the reason for buying my towers.  Jim Salk, who I greatly admire, told me to use an SPL meter.  By the time he finished I thought I need to be an electrical engineer to set my subwoofers up this way.  

I visted Listen Up in Denver who sells REL.  They believe in REL as being a truly seamless subwoofer by the way the speak on connect hooks up.  They also told me if I buy the Rythmik and hook them up using the LFE connection they may not play at all when playing 2 channel because the LFE connection is designed to sense and produce low bass special effects frequencies.  I wonder if he said this in order for me to only consider REL.  I heard a slight difference when he played both the T/9i and the S3 REL with a pair of 704 B&W's, but not an enormous difference.  Not a huge gain in bass.  Perhaps a larger space.  When he disconnected the REL's it seamed like the front speakers pulled back.

If you were in my shoes would you go with a pair of conventional subwoofers like the Rythmik or would go with REL?

I have been looking at the F12 G (the G has the paper cone material).  The lighter weight material supposedly makes the texture of the bass more lifelike and dynamic for music with a very fast response and decay.  
Regardless these seem to be special subwoofers compared to the rest of the pack.  
As for connecting to your receiver definitely connect to the speaker terminals of the amp (parallel with your main speakers).  Connecting to your amplifier will allow the best possible integration with the main speakers because they are each receiving the same signal and sound signature.  
If you make sure that the subwoofers are positioned reasonably - along the main wall, about 6" or so from the wall and not in the corners of the room I don't believe room correction is necessary.  I have measured the respinse of many subwoofers and positioning them reasonably flattens the response which avoids peaks that require correction.  
If this sounds like a good starting point we can walk you through how to adjust the crossover, phase, level, etc. for best possible integration.  

LFE out? did you mean preamp out? LFE out is exclusively for home theater if I am not mistaken (low frequency effects). If you don't use the high level inputs via the speaker cable, then I would either use the extra set of preamp outs on the preamp/receiver, or if only one set, just buy a splitter and run an extra interconnect over to the Rythmiks left or right channel RCA input.

Then on the rythmik you can best set what frequency sounds best to blend with your mains (don't have to touch the mains)