Sabrina X Anyone Add Subwoofer(s)

If you're talking good 2 channel well then I'd have to disagree. 

That’s cool.  How boring would this site be if everyone agreed all the time?

I'm really not looking for more base as much as more authority and clarity.

Well, you’re going to get much more than that.  You’ll find your entire Soundstage will expand in all directions and imaging will improve along with a better sense of air and space.  Then, if you turn the subs off the whole thing collapse and you’ll trip over yourself to turn them back on.  Anyway, that’s my experience in addition to a more solid foundation, authority, and solidity.  

If you are considering spending $5k on a sub that suggests that you could have afforded the Yvette that has a 10" woofer that goes down to 20hz. With the right amplifier probably solid state power, that drives a difficult load 86db and 4ohms, I would hope if you can't get enough bass out of them then there is something fundamentally wrong.

No.  Subs are much more than just getting “enough bass,” and your statement indicates you’ve never heard a good system with properly integrated subs.  Too bad because there’s much more to it than just more bass.  I’d take the Sabrina X and two good subs over the Yvette any day.  Once you hear what subs can add to a system you won’t go back to listening without them. 

Here ya go. This can help integrate dual subs, and at only $400 used it’s $600 less than buying new...

https://www.ebay.com/itm/124769802826

Here’s Kal’s review just FYI...

https://www.stereophile.com/content/music-round-57

Smaller subs .. Do you have specific speakers in mind?

Well, an easy solution would be a pair of JL Labs smaller subs like the E-Sub E112s that’ll still get you down to an honest 22Hz and cost $4600 for both. There’s also the excellent Rhythmic F12PEQ that would also cost around $5k for a pair, and obviously Rel is another viable option depending on your preferred sub connection type. In any case, I’d highly recommend doing any of these options over buying one $5k sub. Hope this helps, and best of luck.

Wilson uses their subs with the Alexandrias, so what does that tell you?  My advice, get two smaller subs — you’ll get much better performance.