Sub/ triangle

I have Triangle Titus also. Wonderful speakers that do what they do very well, but low bass extension is not what they do, so generally a very good candidate to supplement via sub(s).

If you do not care about or need bass extension below 50hz or so to be happy, I'd go sans sub with the Titus and get them to shine as is.

If more full range sound is the goal, I'd focus first what is needed to get sub integrated a best as possible for best sound without "muddying the waters".

From there, I would think about using two matching subs in order to help normalize bass response throughout the room further is bass is good in one listening position but not so much in others. Next step beyond that for same reasons would be to add additional subs, but benefits become increasingly marginal from there.

Yes, speed and clarity is a common attribute of French speakers I have heard, mainly Triangle and Focal. They tend to sound more like planars or eletrostats in this regard than do many other box speaker designs.

Focal makes subs. Probably not a bad place to look, though I do think a lot of speed and clarity issues with subs has more to do with proper setup and integration with the mains than any inherent flaw in most good subs from reputable companies.

I suspect damping is a good thing to consider for speed and clarity with many powered subs. Many high power powered subs use Class D amplifiers nowadays, which tend towards higher damping and associated attributes of speed and clarity. THat bodes well I would think for using many different makes of modern powered sub effectively with speakers like the Triangles when needed.

Bass integration with hybrid speakers like Martin Logan is a common cited issue. Maybe ML might be a logical place to look for a sub in the case of Triangles as well?

I used my Triangle Titus XLs with an older M&K V1b sub for many years with good results up until the sub went up recently, and I have not done much research to date to investigate eventual possible replacement options. SO I am also interested in what people have to say about subs that might work particularly well with Triangle Titus speakers.