Is This A Bad Move?

I have a pair of Large Advents and a pair of ADS L-810s I want to use in the same system. Due to space limitations, I was forced to buy two nice higher-end bookshelf units that coincidentally had the exact measurements needed for the speakers (I took that as a sign I was on the right path). Unfortunately, the bookshelves are not open at the back, and I'm wondering if that will affect the speakers' sound to the point I'll be unhappy. The speakers will be in a horizontal position, which I know works well since that's how I've got them oriented now. I will drill holes in the cabinet backs and install grommets to allow me to run the speaker cables. I will have to make a decision on which pair should be on the lower shelf and I'm leaning toward that being the Advents. Anyone else been down this road?
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mate, i had a pair of Advent legacy II and they were shite and I don't rightly know why anyone ever thought those speakers were worth more than their weight in feathers... but if ya gotta do it, give it a try.... maybe you get some low end re-inforzament from the woofs
Thanks for the constructive response millercarbon….the bookcases have a framing/molding that is exactly 26" wide, then the actual shelf area opens up to about 28" wide. Both sets of speakers are a shade under 26" wide, which will leave about an inch of space around the ends. There will be about 3" of space above the speaker tops, so I can fabricate a piece of material to fit those areas to keep reverberations down to a minimum. Even though this is my 'vintage' system (one of two, actually), I want to do whatever I can to make it sound its best. Your suggestion seems like a good one.
Millercarbon is spot on with his comments regarding your bookshelf.  Interestingly,  Michael Kelly (formerly a/d/s and then Aerial) incorporated Environment and Treble controls on his large Aerial LR5 monitors for the same purpose you are discussing - different set-up situations.  The settings are Stand/Shelf/In-Wall.  I am not sure exactly the effect of the controls (mine are out in the open on stands so I use the "Stand" setting) but it might be worth looking into whether you could use EQ to achieve the sound you want from the bookshelf location.  Most computer sources can accommodate some sort of EQ programming.

I owned the L810s and they sounded great for their time and still pretty good today. Be sure to fuse the tweeters appropriately since it was not unusual to blow them.  I wish I had never sold mine.
disknic, the problem is not so much how the speaker sounds, it is how the cabinet sounds which is dependent on its construction. 1/4" plywood backs are going to resonate like a drum head and other loose parts are going to vibrate and potentially make a lot of noise. 
The best cabinets for bookshelf speakers have no backs at least in the bookshelf section and are solidly constructed with fixed and dovetailed shelves at least an inch thick. Knock down construction is a nightmare.
If you wind up with a few buzzes you can find them by damping various parts with your hand. Once you find them just jamb a little butyl glazing tape in there and that should stop it. You can get the tape at your local Portland Glass dealer or any place that glazes windows.
The large Advents sound best upright on stands. Why are you putting two pairs of speakers in the cabinet? Doing that will ruin your imaging if you care about that.