Aerial 9 need a sub?

Most likely not unless, 1) its used for home theater, 2) you have a very large listening room or, 3) you listen to rock and you really love floor shaking bass. It also depends greatly on what amplifier it’s paired with. You’ll need a high power/current solid state amplifier to make use of their potential.

If it can actually reach 30Hz in-room at loud volumes, a sub isn’t needed if your content doesn’t have deep bass (movies, hip-hop, etc.), like most rock music has nothing below 40Hz. I always run subs in my setups as the content I consume benefits greatly from doing so.

jazz and vocals are what I like. I do play the occasional rock but not very often. I use a BAT VK500 amp.

A properly implemented sub will free up the amp from working to get those power hungry bass tones....and therefore the mids will become more accurate with greater ease.

what sub would you think of pairing with the Aerial 9?

Do you mean Aerial Acoustics? I have the older 7B towers. I never felt like I needed a sub but tried a large, maybe 12" B&W powered sub along with them and did not care for it.

Now, my amp is 250w per channel so it has power to spare even for these (I've heard) inefficient speakers.

In any case, I find the tight, precise bass to be one of the strong points of the 7Bs and felt like the sub muddied things up a bit. Maybe a better sub would have given different results, I don't know.

But even with music like Saint-Saenz organ symphony it will shake the room at moderate volume and still not get muddy.

It looks like the 9 has two more woofers per speaker than my 7Bs so I have a hard time imagining why they would 'need' a sub.

why wouldnt you just listen for a few days / weeks before deciding if you need a sub or not ??

Agreed. And if the Aerial 9 that he is referring to is the Aerial Acoustics 9 then each speaker has 4 7" woofers. 

Another piece of advice to the OP. This may be obvious to you but before buying  sub-woofer spend a good amount of time positioning your speakers within your room. According to what I see about the speakers the bass port is out the bottom which gives you a little more freedom in regard to wall proximity than with a rear port but positioning will have an impact on bass performance. 

My 7Bs have a sweet spot in my room and even with the rear bass port that position is closer to the walls than most people would recommend.