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

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.

The laws of physics remain what they are. You cannot get deep bass out of a small box and small drivers. The more you push low frequency extension, the higher the distortion. Of course technology marches on, but only slowly. In my own case, I recently replaced the LS3/5a monitors in my study by their most modern descendants, the Harbeth P3ESR. The modern sibling does indeed have more extended bass, and better bass as well (and not just bass - these are truly great speakers), but even so, it is nothing like a true full range speaker. Not in terms of low frequency extension, and not in terms of dynamic potential.
If you want deep bass there are only two possibilities. The first is new full range speakers. The second is subwoofers. As it so happenend I have a B&W PV1d subwoofer in my main system, and one day I decided to experiment and combine it with the little Harbeths. The match was easy to achieve and the sound was spectacularly unreal. You think all that deep bass is coming out of these two litle boxes. The extra power from the sub also gave some more dynamics, even though it still did not quite fill the large main listening room that I was using for the experiment.
So if you are happy with your current mini monitors, I think the subwoofer route is the way to go. If you decide to do this, I have two recommendations, and they both concern room modes. Room modes are the real problem of low frequency reproduction, and they have to be addressed if you want a good integration, and want to avoid boomy and woolly one tone bass. I think you would need to do two things. The first is to use multiple (small) subs - at least two and perhaps more, to smoothen response. See here for an introduction: http://www.acousticfrontiers.com/20101029using-multiple-subwoofers-to-improve-bass-the-welti-devanti...
Have a look at the Audiokinesis Swarm system: http://www.theabsolutesound.com/articles/audiokinesis-swarm-subwoofer-system/
The second would be to add room equalization like the Antimode 8033: http://www.theabsolutesound.com/articles/dspeaker-anti-mode-8033-dsp-subwoofer-equalizer-tas-204/ I have one and I am very pleased.
Combine multiple subs with room eq, and you have the best of what is possible in a domestic context.
However, if you are using mini monitors because your room is very small, forget about all this, and just learn to live with what you have. How large is your room?

I nominate williemj for post of the week...or maybe the month!  Nothing more to be said.