REL Sub T1 or T2 Good Match for Dynaudio Focus 140

As a Dynaudio dealer I might be somewhat biased, but why not stick with Dynaudio and look at the Sub250?

The Dynaudio subs are superb imho. They play clean and tight without any hint of boomyness to them. With control at your fingertips to adjust the bass settings makes the Dynaudio subs just about as flexible as you can get.

The Dynaudio Sub 250 is indeed a very nice sounding subwoofer. I owned one and used it with my C1's and if you are able to use the high pass filter option that may work well for you. However, I can tell you that in my house the REL T series is even more invisible and musical than the Dynaudio Sub 250 when used to augment the low frequencies only for music playback. The REL connects via speaker level and the blending is seamless. I love Dynaudio and think they make amazing speakers, but for 2-channel music listening, the REL in my opinion is the better choice here. See if you can try them out in your home first and form your own opinion.

I haven't heard the "T" series Rel subs...

I tried the R205 and liked it a lot. I'm now trying the B3 and like it even more.

These two Rels are the best sounding subs I have ever had in my systems and there have been plenty!

Warning... Shameless plug to follow ;)
Long time Dynaudio owner here... starting with two pairs of Audience monitors which I still own, then Contour 1.3SE's, then Confidence C2's, and currently the Confidence C4's in my main system. I've had great luck with my REL Storm III over the last few years, but am thinking about selling it as my C4's register quite well in the lower frequencies and can over-power my room when paired with the REL.... so the REL has been sitting idle since shortly after my C4's arrived. Let me know if you are interested.