Signature Pro are eons ahead of the pro2900s in terms of audio.. its one of the best headphones available for detail retrieval and instrument separation rivaling the HD800 (IMO); unlike the HD800 they have a pretty tiny soudnstage - S-Logic does not work for everyone's ears. Also, the treble is smooth and accurate on the SP (unlike the 2900s).. But I don't think its a fair comparison either, the Signature Pros are flagship reference headphones while I consider the 2900s just open pro900s (with some driver tweaking - similar sound characteristics but definitely not reference quality).
Comfort wise I prefer the 2900s because of the larger diameter pads and velour instead of leather. Build is about the same for both, but the SP's get some nice little details like sheepskin leather headband and pads + glass name plate. Note that while the 2900s are open headphones their soundstage is nothing to write home about; at the time I also owned the Denon D7000s which are semi-closed cans and felt the soundstage on them surpassed the 2900s. Comparing the 2900s vs the SP strictly based on soundstage though, the 2900s do better.
Edited by Ahzari - 11/28/12 at 12:17am