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

Why not believe the dealer?  He has already assessed your need. The T9 is the largest in the T series. with a 10 woofer. I don't believe the idea that the Rel can't provide bass. It's getting the same signal as the speaker. Just because they can't reproduce the bass doesn't mean the sub can't

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.  

I’m dubious about the need or benefit of multiple woofers, unless you need the floor to bounce, and like banging your head. I had a friend who’s method of evaluating a speaker was to turn the bass up to 11, and the volume up to twice that. He drove me to 11 when he pulled it on my setup.

I have an SVS SB-3000, now ain my multi-media room, and the REL, mentioned above, very recently acquired. SVS advises keeping the settings such that you can’t spatially locate the sub with your ears. I like that advice. Still, head-jarring volumes are possible if it’s your thing. The REL to my ears has a very desirable, more subtle quality about its bass by nature. I haven’t been able to replicate the "boom" with the REL. I’m ok with that.

You’ll have to experience both yourself, I’m not too sorry to say. Have fun, and Best Wishes.

The need for a full range speaker is simply not true. I have a pair of T/9x’s paired with Harbeth 30.2 XDs, stand mounts, rated at 50Hz – 20kHz. They work great! The difference in cost between the 7’s and 9’s isn’t that much, BUT I strongly recommend a stereo pair of RELs. EDIT: Assuming you are mostly interested in stereo audio. 1 is fine if surround sound is the primary application.

You can dial in the crossover to integrate with the main speakers frequency range, whatever that is. The X1’s are rated at 80 Hz – 50 KHz so maybe yours are close?