Not exactly sure what type of ohm load I'd be putting on my amp using a 4ohm cab and a 8 ohm cab. I'm using a SWR 900 and from what I understand, 4 ohms is the minimal it will take. What do you all think? Thanks

I would assume that running an 8 ohm and a 4 ohm would not be good for your head since you would be running at more than 4 ohms.

Using 4 ohm and 8 ohm cabinets together would load down the amp at 3.2 ohms. If your amp's minimum rating is 4 ohm you could potentially damage the amp. I have seen people use amps in this situation and there was no problem. BUT I recently saw a guy run two 4 ohm cabs with the amp bridged at 4 ohm and after the show the plastic rack that the amp was in was REALLY hot. If you want to use two cabs, I would suggest going with two 8 ohms or get another amp that can handle 2 ohms. ccbass71

Impedance really isn't that hard. If the cabs are in series, you just add the impedances. Eg 8 + 8 = 16. That's why most amps/cabs don't have the speakers in series, adding a second cab makes the setup quieter! So, most cabs are in parallel. The general case for parallel cabs of impedance A and B: (A x B) / (A + B) Eg (8 * 4) / (8 + 4) = 32 / 12 = 2.7 However, for the normal case of two parallel cabs of equal impedance, just divide by two. A further note: No matter what you do, two cabs in parallel will always come out lower than the lowest cab. So if your amp can do a minimum of 4 ohms, and you have a 4 ohm cab, you cannot safely add any cabs in parallel.

Your SWR 900 will do 400 watts @ 4 Ohms per side or 240 watts per side @ 8 ohms. This means that in stereo, you could run your 8 Ohm cab @ 240 watts & your other channel could run @ 4 Ohm with 400. Mark AccuGroove.com

If I'm right: Running in mono, you'll find the 4 ohm cabinet receiving twice the power delivered to the 8 ohm cabinet. This can be used to advantage, making one cabinet louder than another, affecting the final balance.

Math . Not my strong subject. I thank everyone for their assistance. Let's add to this, if I'm running biamp mode, I assume I'm using 2 different side's of the power amp which each excepting what ohm load I put on it. 1 side 4, the other side 8. Problem is the 15 is 8ohm and the 10's are at 4. I assumption is you generally want more power to your lower end, which in actuality will be the opposite. But then again, the 10's do have a wider dynamic range then the single 15. Opinion's?

I would say just balance the output of the two power amps until you have a good mix. Or add another eight ohm 15 to your lows. At least you have some options with your amp.