I'm partial to the Bosendorfer Imperial for booming bass sounds (and, in theory, you can actually but one), but the Alexander piano is a very cool experiment.
I've played a couple of Bosendorfers including an Imperial and the best I can describe it is that the lower end of the standard 88 keys produce very clean, solid bass sound and don't "fart". It's less about "booming" and more about just a very clean bass sound.
The extra keys below the 88 do go into bass-fart territory, though. (For those who don't know, Bosendorfer Imperials have 97 keys, the extra ones all going deeper into the low range).
