Basically what Grawk said - near field monitors shouldn't be expected to have significant output below 100Hz. Some have an FR hump at around 200Hz for more apparent "kick", but you won't get the physical feedback that floorstanders can provide. Monitors should excel at imaging, soundstage and neutral FR. Extra bass can be had by carefully implementing a subwoofer, however that is prone to excite many room related issues.
For piano a good 8 incher should do the trick.