Speaker design is all about balancing trade-offs. If you want something, you have to give something up.
The original Silk Monitors feature bass extension that is amazing for a speaker of such diminutive size. And the resulting speaker is super-smooth from top to bottom. Gene DelaSalla of Audioholics called it the best bookshelf speaker in the world. I wouldn't be so brash as to say something like that, but it is certainly one of the finest bookshelf speakers I have ever heard. But the trade-off is lower sensitivity.
The Silk AT provides greater sensitivity. But the trade-off is a larger cabinet and more limited bass extension. The higher sensitivity sure makes it a more attractive design for tube amplification. But at 60 watts, at reasonable volumes, the original Silk Monitors would likely perform OK.
At 92db, the Exotica Monitors and Exotica 3's are perfectly suited to tube amplification and sound wonderful when driven by a good tube amplifier. But they are larger yet again (I wouldn't call them a bookshelf speaker), more expensive and are limited to about 52Hz in terms of bass extension.
So you could look at these three examples of how trade-offs impact a speaker design. There is no free lunch in the world of speaker design. If you truly want something (high sensitivity, bass extension, small cabinet size, etc.), you must be willing to give something else up.
I often get a kick out of someone commenting on a specific speaker design (ours or others). "Why don't you make the sensitivity higher." "Why is the cabinet so large?" "Can't you make it play deeper?"
The drivers you choose determine these sorts of things and are chosen with some critical performance attribute in mind. When you pick a combination of drivers, you get what they provide...nothing more, nothing less. It's all about trade-offs.