Aye, sit on them and take for an extended ride. My back starts hurting after about 10 miles on Kaiser's Allez (54cm compact) too, but it doesn't feel cramped to sit on - a longer stem fitted and a bit of layback on the seatpost obviously help there.
Dodgy back definitely doesn't help though, but it would be interesting to try something a bit more stretched out and see if my back isn't quite as bad.