If your seat is already high on the 56cm frame, consider maybe getting a bigger frame. That's right, I said bigger. It may seem counter-intuitive, but at the same time, look at the frame: The seat post comes out of the seat tube at an angle. When you put your seat up higher (as in, what you'll have to do if you get a smaller frame) it also pushes the seat back at the same time. So, by going to a smaller frame, you could actually increase your reach to the bars even though the top tube is getting smaller. If you go bigger and have your seat further down, you could, in effect, reduce your reach to the bars even though your top tube may be getting a little longer. Sheldon Brown opened my eyes to this on the sizing portion of his website. Not to mention, as cs1 mentioned, a smaller frame will put your seat way higher than your bars. That sounds uncomfortable to me. Rivendell bike works also tend to believe that larger frames make for a more comfortable ride. I started riding bigger bikes and tend to agree. YMMV, of course.
BTW - If you're worried about smashing your junk off the top tube on a bigger bike, consider this: When was the last time you really had to jump forward off of your seat and straddle the top tube? I NEVER HAVE and have been riding bikes for decades. Maybe if you were riding a mountain bike off road in really treacherous terrain, this might be an issue (which is why a lot of mountain bikes have sloping top tubes). But otherwise, for normal riding, you'll probably never have to do that (and if you do, then there's probably going to be more pain in store for you than just the pain that is caused by smashing your goods off the top tube). I wouldn't advise getting a bike that's REALLY HUGE for you, but one that is a little shy of the-bike-shop-recommended 1-2" of top tube clearance would be okay ESPECIALLY if you do find that it makes you more comfortable while in the saddle.