Hey supervij
I would say you can relieve your mains of the lower end duties, and set your crossover higher (80hz maybe a bit lower) as just because they go that low, doesn't mean that they aren't pushing the limits when the get there... Set 'em to small and try out different crossover settings, test with spl meters and chart your FR to see where your the best response is on paper.
Buying a set of speakers these days, the newer trends are headed towards very good performance mid & high range so you don't need to have FR all the way to down low, where a dedicated well designed sub will can handle the grunt work.
Subs are really taking off, and the performance is extrodinary these days, for not a ton of money... some products are really reaching into the depths while still maintaining very good, if not awesome sound quality...
That is if you care to invest in a good quality product...
