Which subwoofer?

https://www.svsound.com/

Very good quality and reputation also offer a lengthy trial period along with free shipping TO and FRO. 

Chuck 

I do own a couple of great but older REL subs (B1, Storm III) but recently acquired three sealed SVS subs, one SB-3000 and two SB-2000 subs (close-out). I'm very impressed with the modern technology.

I owned SL-3s as main speakers for 16 years (still have them) and if your bass quantity is sufficient from your MLs then I'd try to cross very low to add clean deep bass (usually low volume as well) to what you have for a more grounded presentation. Good luck!

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.