Thank you everyone for the informative responses, and apologies for not responding sooner. From the feedback provided, I started to put together a flexible shortlist, and some of my reasoning.
Vandersteen VLR CT - I also heard these with my Belles from Johnny, and though they were awesome, so must be considered.
Salk - A name I've never heard until a few days ago, along with the number of people who suggested, I love number of different finishes, and find it very interesting to see a variety of models at the same price point. I also like the idea that they're handmade, and don't mind waiting a while for it. After all, that's part of what I found appealing with the Belles, hand made in the USA.
Sonos Faber Sonetto II - I've always thought they just look great.
*****Update******
After showing my wife the Salk Songtower and explaining that it is only 8" wide (44.5 H x 8 W x 12 D), a thin tower is now an option. And then it got better. She asked me, "If you spend $5k, will they really be much better?", and then upped the budget to $5k. There's 1 minor catch. As the reason for a bookshelf because the speaker needed to be above any objects directly blocking it (like a chair), I'll need to have the bottom of the lowest driver at least 30" off the ground.
So I'm not sure if I need to start a new thread on a new topic due to the increased budget and now open to a few thin floor standers with highly positioned drivers.
I showed her the Dali Rubicon 5 in walnut and she loved it. I'm slightly partial to these as well because I've been very happy with my Ikon IIs. The salesmen at the time described them as a lot of bang for buck, and I agree, so I'd expect similar with the higher end models.
So is it a no brainer to get a tower, or is there the opinion that high end bookshelf can easily compete with a quality tower in terms of the low end frequency range?