Please suggest bookshelf with best bass response under $4000 pr.

I just found an a'gon ad for monitors with this claim: 

"The __________ are full range Studio Monitors that will
provide the entire frequency range, therefore, eliminating the need of using a subwoofer which cannot be fully integrate with speakers".

I wasn't aware such performance was even possible. Any thoughts ? ? ?  

Why not add a nice subwoofer to your system? The right sub can do wonders!

Neat isobaric (Momentum, Ultimatum) bookshelf speakers have excellent bass for the size. Both can be found used for under 4k. ProAc Response floor standing models also have very solid bass and they sound great with the Wells Majestic. The footprint is no larger than a good bookshelf speaker stand. 

Theoretically a smaller driver with greater excursion can go deep but at the expense of sensitivity.  Ports or transmission line can extend frequency response down as well.  There's no substitute for a big driver in a big box, though.  In my experience you can never get the same visceral impact from small speakers, with or without a sub, that you can get from big ones.  If you're determined to try, the best way to go is active.  Active bass is better.  I consider an 8" driver to be the smallest driver that can begin to give a satisfying punch and go deep enough to not sound wimpy or low resolution on music with siginificant bass content.

Is there some reason you absolutely can't have floorstanders?  Small floorstanders with a pair of 7 or 8" woofers will have a far better chance of satisfying you than another bookshelf speaker.  If you've got your SIlverlines on stands the amount of floorspace taken up will be about the same.  

I'd check out the Dynaudio Confidence C1. I heard a pair once, and was absolutely astonished that so much deep, controlled bass could come from such a (relatively) small stand-mount.