If the receiver has a subwoofer output (RCA out) then you already have a built-in crossover and don't to need to buy another -- unless you are not satisfied with the sound quality, and want to move up to a newer preamp-processor. Anything (receiver or pre-pro) with a dedicated subwoofer output will have a subwoofer level adjustment among the volume controls, so you don't need that feature in the sub itself, nor do you need to buy a separate crossover.
www.diyaudio.com has a whole section devoted to DIY subwoofers. I would not spend megabucks on an 80 pound driver -- those are mainly just for bragging rights, or subwoofer envy if you will.
I bought a "New Larger Subwoofer" kit from www.vmpsaudio.com and have been very happy with it. I don't know if they still sell them as kits or not. It has a 12 inch driver, a 15 inch driver, and a 15 inch passive radiator in one big cabinet, which I disguised as a pedestal for a sculpture. You could also disguise it as a coffee table. My only complaint is that the subwoofer causes nearby plastic objects like my computer tower to rattle loudly during heavy sound effects, which is not the subwoofer's fault. I power my subwoofer with an Adcom GFA-555, the predecessor of the 5500. It works great.
Have fun and give us a followup.