I really do not think most people could hear a difference in a 45hz crossover compared to 50hz.
You could run your speakers full range (seems you don't have a way to cross them) and then just cross your sub at 50hz, that shouldn't sound bad at all. For most crossovers, the actuall crossover point is already -3dbs down anyways. So if you were to cross at 50 hz, you might have about 1dbs extra output from 45-50 hz, and I really don't think that is noticable at all.
But thats just me, i suppose if you keep searching you can figure out how to make a 43-45 hz crossover possible.
But if you havn't listened to it already I would suggest doing that first, it may be just fine.