Adding an Sub that doesn't go well below speakers?

Well, you won’t like what I have to say, but it’s based on repeated experience and measurements.

First, never take a speaker’s -3db point as any indicator of anything that’s happening in your room. The room will make the bass specs utterly useless in understanding what is actually happening. I’m attaching a link that goes over specific examples. Before picking a strategy you should measure what's happening in your room.

Second, the idea of keeping your main speakers going as low as possible because well, you paid so much money for them, is like buying a meal and only eating one half of it. Case after case where audiophiles changed their mind and tried raising the crossover frequency was a huge net positive, but no one likes to hear that their approach isn’t actually going to be optimal. 😪

Also take a look at the AM Acoustics Room Mode simulator and try to keep any speaker and listening location out of the worst/lowest modes.

Out of respect for my own sanity and the OP this is all I’ll say here.

https://speakermakersjourney.blogspot.com/2021/12/room-speakers-eq.html

It does not make sense. 

You would need something like a S510 or better yet S812's to blend with those speakers. I'm using S812's with my Zu Audio Omen Def Supreme, but used them before that with Wilson Watt Puppy 8's. They definitely add a lot of dimensionality to the soundstage as well as adding to the musical foundation.

Marten speakers, nice. 

When any speaker system is positioned in the middle of the room for presentation and sound stage, that room position is usually in the rooms null, choking the speakers potential low frequency output.

Shop for an actual subwoofer that's -3dB at their claimed low frequency output equipped with speaker level inputs and offers some modicum of equalization, or consider DSpeaker or MiniDSP.  

Locate it in your rooms loudest bass mode.   

The biggest problem is the best position for main speakers is often the worst position for bass. You may not be getting the bass your speakers are capable of when in the best location for other sonic aspects. Subs allow you to position your speakers in their best position and put the subs where they work best. Using the small sub you are considering in the correct location will probably make an instantly noticeable improvement in your reproduction.

Plus if your subs have a high pass function(not all do) they will significantly reduce the load on your main speakers and improve the upper bass and lower mid-range.