Need a good high-pass Xover for Sonus REL

I would be one Audiogon member to disagree... introducing a crossover into the signal path might actually affect the resolution and the magic of the SF Concertos. Are you unhappy with the current sound?
If you have bass problems, I would first recommend investing in a sound level meter and a test cd with specified bass frequencies to see/measure if you have excessive bass or bass suck-out at certain bandwiths, and play with placement of- and the crossover setting of the REL. You can pick up a Radioshack SL meter for about $50 and a Rives Test CD 2 for $24 that is designed to be used with the Radioshack meter.
If you really have problems that require a crossover, I would look at the new equalizer from Velodyne, but that would be a considerably more investment.

And I join Arni and second the "disasgree" vote. I just went thru this discussion in great detail on another thread and offline. Let the Monitors' bass roll off naturally. Measure it if you can -- at what frequency does the bass START to fall off in volume, and at what frequency is its volume down by HALF. Set the sub's upper limit (where it STARTS to roll off) 10 to 20 Hz BELOW the half point frequency of the main speaker rolloff.

Here's an example:

Let's say the monitors' bass SPL (sound pressure level expressed in dB) starts to fall at 80Hz (at whatever you measure the SPL) and it's half as loud (ie down in SPL by 3dB) at 60 Hz. Then for starters, you'd want to start the sub roll off at approximately 40Hz (an equal amount the other side of the 60 Hz midpoint frequency) -- this is assuming the sub is putting out the same SPL at 40Hz as the monitors are putting out at 80dB.

The object is to have the two curves cross at 60Hz at half their SPL so that when added together the two curves form an (ideally of course) flat line.

IMO, and in that of the people at REL apparently, there's no reason to "clip off" the low end performance of the monitors. Doing that shouldn't make them "work better" if they've been designed well to begin with.
.