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, soli

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).