saddle numbness

In my experience the main factor is seat height. Whether that means changing the nose angle of the seat, or lowering or raising the seatpost to a proper leg extension height. A properly fit bike shouldn't give much if any problem.

That said, I have some historical hamstring issues that force me to keep my seat slightly lower than I'd prefer, and my nether realms pay for it. To compensate I got a profile design Tri Stryke based on this review.
http://www.bikesportmichigan.com/reviews/saddle06.shtml

If you tend to ride forward in the saddle in aero position and/or are are higher-weight rider, then look into a tri-specific saddle or a little extra gel. My only word of warning is not to go too deep on the gel and padding. If the saddle is extra gel padded and your shorts are heavily padded, then you can actually add pressure down there. The tri saddles are meant to be ridden in trishorts with less padding than your classic chamois.

The other common answer is the "anatomic cutout". Watch out for these. Most are no help at all since they collapse in on you and the pressure is right where you left it. The profile design has a reinforrcement to prevent that.

Happy riding,

Harry