One of my old amps finally decided to bite the dust after 4 years of use and I am replacing it with an upgrade. It's going to be a QSC RMX 4050 (4000 watts). This brings my arsenal up to 18,000 watts.
Anyone throwing a small concert any time soon??

Sweet. I love lots of power...

Also aquired a 32 channel Mackie mixer recently for all your' live music needs and in the process of building another pair of wedge monitors. I'll post pics of them under construction later.

Was it one of your Behringers that went out?

Would you consider 4 years a normal lifespan considering how much you use them?

Yes, it was one of the Behringer amps, but it was a B-stock model that I bought on e-bay. All my other Behringers were bought from musician's friend, but just to be on the safe side all my subs are now driven by Crown's and a QSC.
The problem ended up being the triac that protects the output of the amp from DC current. This makes perfect sense because it was after a long night of drum 'n bass and I was running those amps hard. Lower frequencies more closely resemble DC current but with it switching on and off enough, the triac never has a chance to fully engage. It's like driving on the highway with the clutch half way in.

nice aaron.

how muc would it cost you to build two dual 21" cabs and two tops?

I would actually not recomend the 21" subwoofers. I've burnt up four of them in about 18 months. I was initially drawn to the idea because of the huge amounts of air that they could potentially move but this ends up being their downfall. They are great for mid-bass frequencies such as kick drums, but poorly reproduce lower frequencies for two reasons both relating back to cone area. The first is simple fluid dynamics. The wave front is so large that it requires more room to essentially "unfold" and you don't get the full dynamic of the bass anywhere in close proximety of the cabinet. The other reason is electrical. Because of the added weight of the larger cone as well as the extra resistance to flow the air in front and back of it introduce to the voice coil, their high efficiency rolls off steadily under 80 Hz and to top it off, these low frequencies have a reduced power handling capability which is exactly why I buned out voice coils on four of them.
I have ended up modifying the cabinets I had the 21"s in and replacing them each with two 15"s.
On the bright side, I burn 'em so you don't have to.
As for how much it would cost to build cabinets, this is going to depend on the details. I would say to figure at least $200/cabinet plus the cost of drivers, connectors, grills, ect.

yeah, were thinking about investing in an edj soundsystem. something with lots o bass and great midrange and highs

The best way to go about that; Give me an idea of what type of size range you are thinking about, both physical size and wattage, and what is your' budget? I'll put together some concepts for you with a price tag and you tell me go or no.

To be completely honest with you though, I would be very aprehensive about investing in PA gear. It gets very expensive very quick and there are lots of resources for rental including myself that are similarly cost effective without the large long term investment. There are a slew of hidden costs associated as well such as a variety of cables and replacing blown drivers. If your' system is large enough, you'll need transportion for it. Then there is setup and teardown at every show on top of the long list of things that need tending to as a promoter.
Myself for example, I've been renting my gear for about five years now and have yet to get out of the red. I only continue with this business because I enjoy it and because there is a faint light at the end of the tunnel. I'm just about where I want to be with gear and I've eliminated most of the rediculous expenses such as replacing poorly engineered speakers ect. I've learned the hard way that shortcuts are rarely benificial in the long run. If all goes well, I will have paid off my debt and start making money on this gig in a couple years.

With that said, I'll still help you put together a system if you are set on it. If so, you should give me a call and we can sort through the details.