Bookshelves plus sub 2.1 certainly works out less expensive if you want full range.
I think the Phil 3 is your best option for a full ranger, but that will run you $3500 plus shipping and the form factor may not work for your room.
As far as integration, if you are willing to put in the time and have flexibility for locating your subs, you should get good integration.
However, you should also consider that with full range speakers, you have a two-fold requirement. You need to position your speakers for their best sound stage, but that might not be the best place for your 20-80Hz bass come from.
Since you have a sub now, you might play with a sub crawl and see how the subs would sound in the area you would expect your speakers to be located.
PS - Next time put a $ sign in front of the price. I was expecting to find out what a "model 2500 under floor speaker" was and why in the hell anyone would make a speaker that goes under the floor!