Which subwoofer?

The REL t5x goes down to 32 hz and my Martin Logans go down to 34 hz. Will I notice the additional 2 hz of bass energy?

The short answer is yes you should, but probably not for the reasons you are thinking. The reason you will notice more extension is because the REL and any other powered sub will have a separate volume control. 

Your Martin-Logans like most every other quality speaker manufacturer is more than likely trying to achieve as flat a frequency response curve as they can. The problem is our ears do not perceive frequencies in a linear manner, so at average listening volumes the midrange is going to appear much more present than low bass. Depending on what type of music you listen to, you may actually want to have the low bass several db louder than the mid frequencies to achieve the sound you are looking for. Since all levels are fixed in the ML’s you would need some type of external eq. to make that change which may or may not work for you.

With a separate sub you now have control over the volume relationship between the low bass and mid/hi frequencies. The problem I see in your case with the low roll-off point and the passive radiator of your ML’s, is integrating a sub at a high enough crossover point and volume level with out some type of external crossover will probably be very difficult.

I would recommend something from the JL Audio “e” series and if you are willing to add an external crossover, then pretty much any sub should work.

There are hundreds of models of subs out there, and people get good results with many of them. Other people never get them set up and working as they'd hoped.

The key to getting good results is good integration. That means positioning, level matching, good crossovers, phase adjustment, and perhaps DSP to smooth the response. It rarely will work well just to plop the sub(s) down next to the main speakers and hope for the best.

I agree with those who say 2 subs will work better than 1. This smooths the in-room bass response, but again, that depends on positioning. As to setup. I’m impressed by the app provided with many of the SVS subs (e.g., SB-1000 Pro). The app lets you set level, phase, and up to 3 DSP points from the listening chair.

Since results depend so much on setup, my main recommendation is not to buy anything that can’t be returned if you don’t find it satisfactory.

I have 2 Rel t/5 in my small listening room and they are AWESOME!

https://www.parts-express.com/10-Wireless-Subwoofer-Package-with-Dayton-Audio-SUB-1000-300-5792

2-4 will work just fine. You'll be good to 25 or lower and cut the ML at 40-50.

You will enjoy FAST clearer BASS. 

That Mac has tone control, use it to get things right..

Room treatment, in a small room is a must... Thick floor to ceiling curtains work VERY well and help to dampen the room by opening and closing them. NOT just the curtain,. Windows and DOOR can be opened to release pressure too. Curtains will still dampen the opening..

1-400 watt 8 ohm plate amps get LOUD..  I use to hit the bump stops with a 12", 15", and a 15" passive drivers in 6 cf boxes all the time 125/250 watt plate amps. 18hz to 250 hz clear as could be..
VMPS RE- powered subs.. great value too.

My design was and still is very close.. I like using passives to really tune the blend the subs with the BASS. I use to use Mor-tite weather seal and pinch putty to get it JUST right.. (stays soft for 20 years)

NOW with DSP "perfect" is almost doable.. if you like it PERFECT.. :-)

I have a terrific Velodyne HGS10BG (10" woofer in black gloss) that I've had for years, but it has just sat, unused and unsold because it's extremely heavy and would be very expensive and a pain to ship.  It's a high-gain servo model that can keep up very well with the electrostatic panel.  I also have a pair of 18' AudioQuest woofer interconnects.  But I don't know how to give you my contact information; I'm afraid it would deleted by Audiogon.

I used this subwoofer with the Martin Logan Monoliths (one of their very first models), and it worked great.  The Monoliths were just that---Monolithic and HUGE!  I think the electrostatic panel was 4' by 2'.

After I sold the Monoliths, I had no use for the Velodyne, so it's just sat unused.  But it works great!

Look up the Martin Logan Monoliths online to see what they looked like.  Also look up the Velodyne HGS10.

If you know a way to enter contact information on Audiogon, let me know.  I would sell it all to you....for 1/2 of what I paid (or we could discuss).