how do you know if a pair of subs have built in cross overs , some one gave me a pair of sound labs , marked 1000 watt on the back. suppose there 500 rms each. would i need to use a cross over with these if i run them with a pair of normal cabs?

My experience was always that bass bins could handle all frequencies so didn't require a x-over whilst tweeters couldn't only handle the higher frequencies and so need a x-over to filter out the low frequncies.

You should only require a x-over if you intend to run the bass bins and the other speakers from one side of the amp and you need to show the amp the correct resistance.

If your bass bins are 8 ohms and your top ends are 8 ohms, you can just link them together without a x-over and it will give a resistance to the amp of 4 ohms, which should be the optimum resistance for your amp.

For the best results you need to run them with a crossover at some point in your system otherwise they won't be doing the job they are built for...

The easiest way to find out if they have a built in crossover (known as a passive crossover) is to pop one of the drivers out and peer inside. The crossover will be a small collection of electronic components screwed at the back.

It's best to use a seperate electronic crossover and seperate amp if there's none fitted in the cabinet otherwise you'll be throwing lots of your amp power away trying to reproduce frequencies the bins cannot handle but will still consume.