[Mayhem]

Mr Sound is correct in saying that you should always have some headroom in your system. Many people think that speakers only blow by over powering them. Whilst this is often the cause, you will hear the distortion (or see the clip lights) and (should) drop the gain.

So - in saying that, it would seem that your best option is to use a single set of JBL speakers, as they are 250W each and your amp is 250W (I am assuming per side). Also - Is that Nominal or Peak power?

If you do ever get another amp and what to hook up more speakers, the formulae for working out the impedance when connecting speakers in parallel is:

1/Rt = 1/R1 + 1/R2 etc

Therefore, if your speakers are both 8ohms your total resistance (impedance) is equal to 1/8 + 1/8 which is 0.125 + 0.125 which equals 0.25

1/0.125 equals 4

As Mr Sound correctly pointed out if the speakers are all the same impedance, then you simply divide the impedance by 2 if you are connecting 2 boxed in parallel. However, it is good to know how to calculate them in case you either have mismatched impedance boxes or are building split systems.

Before you connect up two different makes of speakers, always check that the wiring is in fact the same. There are certain manufacturers out there that do not follow the Pin 1, Pin 3 arrangement that is most common in XLR input/thruputs. A multimeter will tell you which 2 pins are used but you may wish to open the box to ensure you have the polarity correct (although the lack of bottom end with give you a heads up if you get it wrong). If you cannot guarantee that no one has been inside the boxes in the past, I would suggest opening them up and having a good look inside. Besides, this is a good opportunity to clean out any crap floating around inside.

One other thing, the use of passive cross-overs will also cause impedance changes.