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?
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.