Subwoofer recommendation and wisdom

yes, one of the popular myths is that two subs is better than one.

@parkergetdean  Yeah no, it’s not a myth at all and very well documented in research and by the experience of many here.  Todd Welti is a foremost expert on subs and you can read his research and thoughts on why two subs are significantly better than one.  I agree with @jastralfu that if you can only afford one decent sub then do that with the intention of adding a second at some point. 

Hello @stuartbmw3 ,  one question, several answers.  Yes no doubt two is better than one, and I am a huge REL fan, and have pairs of Carbon Specials and Carbon Limiteds.  But sometimes you can get lucky and one sub can work pretty well, especially if you can play with its location.  You don’t node if you don’t node. In my family room I can unplug one and it still sounds great.  In my basement two subs really evens it out, and I still got some booming I handled with corner traps.  Get a good sealed sub and go from there, with the thought you may want another one.  I avoid the DSP complication and appreciate my music least processed.  I do run my basement subs with REL wireless and that allowed me to move them around very easily until they didn’t boom.  It was not where I initially envisioned them!  No one can draw a diagram that is sure to work unless maybe if you have a very very regular room. The REL wireless works extremely well.  And, once dialed in, I find no need to mess with DSP and sub volume from my chair.  Oh, and people that say their speakers don’t need subs, generally have not heard well integrated, musical subs.  They add physical touch, recreate the real music, and support soundstage and imaging; especially two :)

PS - Also try the AM Acoustics Room Mode Simulator, it's amazingly good at visualizing your issues, if any.

The biggest issues with single sub solutions are big, narrow peaks which define the sound.  The peaks can be so bad, up to 20x more power than non-peak output, that you can't turn the sub up.  For ideal integration you want to measure your room and have the ability to clip the peaks appropriately. then you can raise the volume and have what the  OP is describing. 

Also, often  beneficial to plug ported main speakers.  

sometimes wish I had a bit more foundation on the bottom end

 I don't want sledgehammer bass

You are normal. Music first normal. 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.

Yes, two free lunches, two Ferraris, two world records are always better than one. But when you don't want Mick Fleetwood in the room, 2 may not be a must to make you happy.   I can highly recommend the KEF kc62, very friendly to setup.

Depending on the room modes, do a bit of research and you'll find 3 subs is a big improvement over two and two over one. 

Good answer here: https://mehlau.net/audio/multisub_geddes/
AI summary:

# Multi-Subwoofer Setup: The Geddes Method

This technical document explains the acoustical principles underlying multi-subwoofer systems and presents Earl Geddes' evidence-based calibration methodology.

**Acoustic Foundation**: In enclosed rooms, sound reflections create "modes"—frequencies where standing waves amplify or cancel. The Schröder frequency (150–300 Hz in typical rooms) marks a critical threshold. Above it, abundant modes and human hearing's directional capabilities mitigate problems. Below it, sparse modes create problematic peaks and dips, causing "booming" or absent bass. Since humans cannot localize frequencies below 80 Hz, multiple subwoofers distributed throughout the room can smooth this response without audible source indication.

**Statistical Principle**: Geddes' key insight treats bass reproduction statistically: each additional independent subwoofer reduces frequency/spatial variance by approximately 1/n (where n = number of subs). This means the second subwoofer yields significant improvement, the third offers moderate gains, and a fourth shows negligible returns. Critically, subwoofers must be *spatially separated*; proximity creates correlation that diminishes benefits.

**Practical Setup**: Only three well-placed subwoofers are needed: one near the mains (corner), one mid-wall (not corner), one flexible placement. They require only basic controls (level, crossover, phase) and modest power—10" drivers suffice.

**Calibration**: Using measurement equipment, systematically optimize each subwoofer sequentially, adjusting gain/phase/crossover while spatially averaging response near the listening area. The result should achieve approximately ±2–3 dB spatial uniformity across the bass region.