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?
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.
Thanks for the suggestion, mijostyn….I'm using the bookshelves because I just picked up the Advents a month ago or so and don't want to ditch the ADS L-810s. I won't play them together, just one pair at a time. The units are handmade, not knockdown, and I won't be playing music at a loud enough volume to excite any buzzes, hopefully. The shelves are secured really well to the cabinet walls with metal hardware, which should help. They were made by Thomasville Furniture here in NC, which is no longer in business (along with many other furniture companies). The room doubles as a den/office, so space is at a premium.
If you can get just one sheet, OC703 one inch thick, cuts with a knife and you can fit beautifully around and behind. This will kill all sound and most vibration. Its a yucky yellow fiberglass but wrap it with the right fabric and it will look pretty darn custom professional. Wouldn't need much and the whole thing would be crazy cheap yet effective.