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

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.

@hilde45 

I agree 101%. The point isn't so much to lower the frequency response. It's to improve the overall low frequency response in your listening space.

Although the placement of your main speakers may be ideal for the highs and mids, chances are that the low frequency can benefit from the addition of a pair (or even better four) subs. And don't simply plunk a sub next to each of the mains. Any low frequency response problems will be linked to that spot in the room, so the subs should be placed where they perform best, instead of where they look good.

My KEF Reference 5’s supposedly go down to 25hz -6db with the short port tube inserted and I only have one sub about 6’ behind my listening position.  It’s really all I need.

All the best.