REL Subwoofer to match monitor speakers similar to Raidho X-1s?

60-80hz and below is sub and not directional. What does your mains roll off at?

Look at the specs to see if it will work. When ANY salesman recommends ONE sub I take notice. 1980s maybe. NOW 2 subs at least. Few people can set up a single sub for a whole room. Usually because the room needs to be treated first then add the subs. Speakers are not a solutions for poor room acoustics. Placement of the mains and a DBA mentality makes for great vs "I bought a sub". I'm not a huge fan of REL. I have socks older than that company..

They still couple their boxes to the floor. A real bass head knows better. There is no doppler affect on springs and speakers. Total BS to sell worthless spikes. They look nice they work horrible. GRs OB servo or Rythmiks if you want super accurate un smeared sub/bass extension all the way to 20hz.

I try to match efficiency of sub to mains to. Horns with horns and don't try to match 84% efficient subs with 95% mains. If you have a sub remote it kind of works..

My mains are 92% @ 8 ohms and the Subs are 90% @ 12 ohms. They match perfect all the way to the top. There is no bloat because of the servo system on the bottom shelf 100hz and below.

They make it easy these days. You have to work hard to screw up the newer TECH, even if you're just working with DSP.

["the speed of the sub"] ?

Is this your first subwoofer?

You might want to ask your dealer where on the REL site does it give the subwoofer speed? 

Many modern subwoofers have the adjustability to better integrate with a wide variety of speakers than simply crossover and phase. Good luck with your search.  

Thanks to everyone for their input.  Obviously I have a lot to learn.  What I am most certainly NOT pleased about is that I've sent REL 3 emails and left one voice message and not gotten one response.  That in itself might be reason to look elsewhere.  @Artemus_5, good point.  The guy certainly steered me right with the other stuff, but they will only recommend what they sell.  @pingstonesmile, alas due to the WAF, there will have to be compromises made.

T9x would be great, maybe even the t7x.  I had a rel s2 sho and it was too large for the totem arros that I was trying to pair it with. The reason I say this is because the sub would go from too quiet to too loud with 1 click of the volume control.  
 

A pair is going to sound better too, so 2 T7x’s will be better than 1 T9x.

RELs are in deed “fast” which translates into being very articulate. I listen to a lot of music with low frequency information and I have not heard another sub that has the same articulation.  RELs are also a sub bass system so you are slightly correct eith your assumption.  From my experience, SVS subs that I have had, have more impact in the 60hz on up range.  
 

You could look at SVS, Rythmik, and Velodyne as well.

if your dealer has a T9x on hand ask if you can demo it!

Thanks @b_limo.  The good news is that I did hear back from REL finally.  Regarding matching with my speakers, part of the problem is I really cannot get any info on the Eben X-Streames in terms of their specs.  I just know they are like the Raidhos.