I was thinking and just realized that I might have have bought a 8ohm sub. Can I use just one 8ohm sub or is it bad ?

 

You can definitely use an 8-ohm sub (EASY load on the amp). But you will get half of the power out of the amp, compared to a 4-ohm sub.

 

The output will be lower but if the amp is bridged it will be a lot cleaner when pushing hard. But don't expect great output without a big amp with an 8 ohm load.

 

x2 8ohm speakers wired in parallel giving you a 4ohm load is what you would be looking for, not one 8ohm.

 

I know the alumapro is a good sub but would I be better off sending it to a better place and getting a new 4ohm 12"? Also is there any way to tell what ohm ratting it is ? can i use a multimeter?

 

[You can use a multimeter, just set it to DC resistance and round to the nearest ohm b/c the meter will show the resistance, not the actual 2/4/8 ohm impedance, which is calculated slightly differently. To verify, I'm not familiar with the backside of alumapro's but if there isn't any boot over the raw magnet, or if you can access the magnet, the impedance should be stamped somewhere on the side in a "really hard to see ink". Use a light and some reflection manipulation to find it.