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.