I agree with Baron's post about cleaning up your main cabinets first. As was mentioned, start with a CD player and your main cabinets. Don't worry about how much bass the main cabs put out as you will be adding the sub for that. Most of the Carvin boards have some sort of EQ on them so use that to try and get a decent sound out of them. Once you are generally happy with your mains, you can get an active crossover. In addition to the already mentiond brands, you can also look for a used TDM crossover. I have bought several of these used, and then put them in my own rigs for years running hundreds of shows without failure. I have found them to be very reliable, and they are very simple to operate.
Now here is how I calibrate a system:
To do exactly what I do makes an awesum amount of noise, so I am going to give you the 1/3 power version:
First get a CD you know very, very well, and set up all you gear in a "club" scenario (Mains set up like they are in a club, and you & the board set up across the room facing the mains.
After you hook everything up, turn on all the gear, and then turn the amps all the way up, and turn all the inputs and outputs on the active crossover down to 0 (-infinity, whatever).
Next set your subwoofer crossover frequency(90-130Hz), and turn the input dial on the crossover to "0" or "Unity"
Start the CD player so that you have signal coming into the crossover. Make sure the output of the board is between "-10" and "0" or "unity" . In a full throttle calibration you would have the output of the board running at "0" or "+1" or "+2", but like I said this would be way, way loud, so unless you live way out on a farm, I don't recommend it.
SLOWLY turn the low frequency output knob on the crossover up until you hear the sub start to work. If it gets too loud for your environment simply go to the board and lower the CD players level. You want to get the Low Frequency Output knob on the crossover to "0" or "Unity".
Once you have done this, start turning the "HighFrequency Output" knob on the crossover until you feel the highs match the lows. Again, you can control you overall volume with the channel fader on the board. When you are done you should be able to maintain the balance of low and high frequency output wether the PA is running quiet or loud.
IF you can do a full throttle calibration (i.e. you have found a place to make noise that won't get you arrested), then let me know and I will tell you what I do in that situation.