Oh this is one that's not a simple answer situation indeed. So much here depends on the gear being used, and the application, as well as the skill and knowledge of the person setting it up! I can't understate that last part enough.
ONe thing to consider, would be whether you are only doing 2 channel application, or multi channel HT/music set up. The latter tends to work best crossed over through some kind of crossover network, where the sub is playing the demanding bass info, and everything else is handled by the monitor.
I've heared a lot of people's systems for 2 channel, where it's better to play a full range speaker "full range", and play the sub in parallel. Usually however, this set up works best for the less demanding bass and heavy dynamics stuff I think. Surely most any passive speaker played full range for movies needs to have the bass chores taken off it's shoulders, and carried by the active powerd sub(s). In the case of DD/DTS movies, you're largely going to be better off crossing over even large towers as "small", playing the bass to a sub bellow about 80hz(also note proper sub set up for integration and localization is mandatory). THX recommends this situation, and it works very well set up correctly. And unless your speaker are active, and play low with bigger bass drivers, or you have active subs at least in your main speakers, playing your speakers as as "small", with the sub playing bellow that is the way to do it for DD/DTS movies.
With lots of music however, given the quality of the gear and actual set up appication, alot of people may get overall better results playing the speaker(bookshelf or floor stander) full range, and letting the sub play in parallel up to where the speaker roll off. If the speaker playes down low enough(like a lot of floor standers), you might have some overlap from where the sub plays up to and where the mains play down to. This is a compromise probably in ultimate "ideal terms", especially when you consider most subs adjustable crossovers start at like 40hz and go up! If you're full range speaker plays down to say 25-30 hz, you will be overlapping. But, notthings perfect. And there's likely an accepable balance to be had there.
Still, I find that for most of my needs, using either full range or bookshelf isn't that critical to match with a sub for most applications. I do prefer the smaller monitor with a sub, but I can get great results with either. They just take different care a bit. Either way, I'm crossing over the mains as "small" through a pre/pro for HT, and maybe heavy rock, techno, whatever. Dynamic bass heavy stuff does better through an Active sub bellow 80hz in my oppinion/experiences, and I don't care about "ultimate sonic purity" when listening to that stuff..so I don't mind enlisting the help of my digital pre/pro for that kind of music, so it's a trade off. But dynamically, the sound is more held together, more effortlessly dynamic, and distortion free when using the crossover. Again, everything still needs to be set up right. And if someone's not experienced in setting things up correctly (most aren't), then playing your speakers full range, and trying to fill in the bottom a bit with a sub might be the best scenario.
I've seen my share of set up's with small monitors and full rangers a like, where the sub isn't set up well in relation to the speakers, and it's better to have the sub maybe "off" for music. Alot of why subs get a bad rap from audiphiles, stems dirrectly from poor set up, even more so than quaility of product!
I would mostly recommend full range passive speakers for people who are going to listen to more full range music, and want to keep the ultimate sonic purity and full range sound together as simple as possible. Then they can use the processors internal crossover set to "small" for movie applications, letting the sub do bellow 80hz. I find this the best for dynamic capability.(like THX). I do however think for most peoples applications and high end music tastes(if that's your gig), small quality monitors on stands might be the best way to go, even played full range in parallel to a sub filling in the bottom, or without. Then, for movies, you still cross over to "small"(80hz or so), and let the sub play bellow that. Or, basically even play the small monitors just by themselves for music, as with most types of music people are likely going to listen to, you might not need the sub. INfact a lot of small monitors will still play respectably down to the high 40hz reigion just fine. I happen to prefer the smaller monitors for quality, price, flexibility, immaging, and convenience often. But again, you can get excellent results either way if you know how to set things up.
All in all, I think either full range or bookshelf can do excellent with a sub if set up right. But I tend to prefer the flexibilty of the smaller monitor, as I'm still going to cross over even the larger speakers as "small" when it comes movie time.
Hope this helps
