Anyone Successfully Go from Floor Standers to Bookshelf Monitors w/ Subs?

I've just taken that route. Had large OBs and changed to ProAc Tablette with two large sub-woofers. This is filling a 15' x 32' x 8' room that opens with an extension of 16' X 10' . 
It fills the room quite a bit. But only when the subs are on. The small bookshelf speakers will not cut it all by themselves. 
The speakers are fed via a 20 watts class A tube amp. The subs are 15" with 250 watts amps each. One per channel. Works for me. 

From Mag 1.7i and others to Proac D2 stand mounts and WOW- Also have a pair of James Loudspeaker Subs for when you want a little more bass-=-= Those Proac bookshelf speakers are special-=-

I just went from standmount/bookshelves - all i've ever had - to my first set of floorstanders (F208's)...
The thing i've noticed most is with the big speakers, the performance sounds bigger... Life sized human voices and instruments.

Being a car guy, I like to compare it similarly...You can have a 4 cylinder turbo that makes as much torque and horsepower as a big liter V8 or V10, but it will just never ever feel the same.

For more than 15 years my speakers were Infinity Kappa 8.1s which are phenomenal towers.  I struggled at first and tried a few different smaller speakers before I developed the ones I use now.  They move a ton of air and are rather extraordinary.  

There is a big difference between some of the more modest models and what you get from more elite speakers.  I love mine (Verdant Audio) but you should listen to Wilson Benesch's like the P1s or Vertexes, Magico A1s, Focal Kanta and Sopra #1s or even Raidho's monitors.  None are cheap but they sound big.  

@OP, lots of recommendations for this speaker or that speaker but to directly address your concerns :

" I am concerned about the ability to fill the room" and
"We don't want to take a backward step but can live with a sideways step"

@spenav mentioned some good points but disagree that room correction is necessary.

Without going into too much theory, know that it is entirely possible to assemble a stand-mount speaker and multi sub setup that will outperform even very expensive and huge floor-standers.

Bass from a pair of floor- standers will never energise a room smoothly simply because the speakers are placed where they can best create an image and sound stage. If the speakers were located where the bass was given priority then for sure there would be no stage or image apparent.

However, when separating duties, the monitors can be easily positioned in the optimum spot for imaging and the subs can be positioned where they integrate seamlessly. It takes 3 or 4 subs to achieve best results. You have 2 subs (from your post?) so get 1 or 2 more. To experiment perhaps borrow a couple of subs, even a pair of 8" subs will do as they are used as tuning devices. With this approach no DSP or room correction is necessary.

I encourage you to read documents and articles by Earl Geddes, Floyd Toole and others on Distributed Bass Array (DBA)

The multi-sub approach and some bass traps optimises bass performance and this really needs to be heard to understand. The peaks in the bass frequencies are reduced and the nulls and partial nulls are filled in some. Nulls, which I am sure you understand, contain no information. It's MIA. EQ and DSP can't manufacture this info.  Going this route is truly transformative and would be no sideways step. Think huge step up with room filling sound, probably better imaging from monitors with a narrower baffle and detail not previously heard.

Moving air is the job of the bass drivers. Sure. Just place them in the right spot.