Subwoofer recommendation and wisdom

It’s not easy to setup a sub to accomplish that fuller sound. Setting up 2 would be a giant pain. It would absolutely mute the beautiful lower midrange of your Harbeth. Yes, you can play with gain and crossover, but if you set it too low, it will hardly do anything for you. I spent years with 2 subs and now I am happy with one.

Sorry but this again is nonsense.  First, your speakers go down to 50Hz so you’d likely be using the subs crossover below 60Hz that is well below the midrange.  Most people who can’t dial in subs properly skip the critical first step, which is to first identify the best spots in your specific room (the crawl method is one way) so the subs work with your room and not against it.  People who don’t do this often end up fighting a losing battle and blame the subs for their problems, and this is even more important in a square room like yours that has its own special problems.  Then properly setting volume, crossover, and phase, which is also not hard if you just know a bit about how to do it right, and sadly many people get this wrong too.  I have links on sub placement and optimizing adjustments if you’re interested. 

Have to agree with @soix, setting up two isn’t any more difficult than one if done properly.  As far as muting the midrange of any speaker, that’s not true if the sub is set up correctly.  It will complement that midrange, possibly even making it sound better, by filling in lower frequencies.  Do it right and you don’t need to shoot for sledgehammer bass, just good, clean, musical bass. 

It will complement that midrange

Nope. The sub's job is not to complement the midrange. 

https://www.youtube.com/watch?v=U3q5RHRVEV8

Also, bullying often works, so yes, get 2, 3, 4 subs. I will stay quiet.

@parkergetdean  When I say complement I mean something like this:

While midrange provides the specific notes and articulation of instruments, the subwoofer provides the sense of space and scale and allows each component to focus on what it does best, resulting in cleaner, more accurate sound across the entire spectrum.  Interesting video that seems to make the point that a sub can muck up the midrange, but he states that is if it’s not set up correctly.  I’m not sure it proves your point.  Anyway, sorry if you’re feeling bullied, that’s not my intention at all.  If you’ve found that you like one sub and that works for you then I'm happy for you

First, I recommend watching Nemo Propaganda's most recent posting on YouTube on the Rel s/550.  He has reviewed more subs than anyone (his statement, not mine), and places everything into perspective.

Secondly, I had one shot to buy a sub (WAF), and I didn't have the luxury of playing around with different brands...I went straight for a Rel S510.  That one sub completely transformed my system in ways I never thought possible.  Expensive, absolutely, but zero complaints about the sound quality nor integration within my system, which was seamless.

After 4 years of using that one sub, I decided to add a second Rel S/510.  I can tell you first hand, the one sub provided 80% of what two subs can provide.  But,  having 2 subs truly completed the soundstage and enhanced the realism by quite a bit.  So, yes,  2 subs are better than one but not for overall output,  but rather on completing the soundstage and making the sound more homogeneous and realistic.  It's still amazing how well one sub did though...I didn't even notice the gaps in the soundstage until I added the second sub.

Lastly, if you're going to get one sub AND if it's going to be a Rel, get the absolute best you can afford within the Rel Serie S line... you'll thank me later.

Good luck.