The Bösendorfer 290 does not extend a full octave below the standard 88 key piano. The 88 key piano goes down to A0, the 97 key Bösendorfer 290 goes down to C0, approx. 16 Hz, an additional 9 keys, a little more than half an octave extra.
http://www.boesendorfer.com/en/model-290-imperial.html
http://www.boesendorfer.com/en/model-290-imperial.html