Is there a way to find out the power a sub can handle without knowing the specs? Reason why I ask is that I built a sub out of old subs and it works but I dont want to blow it up by giving it too much power.

 

The biggest limitation is usually the thermal power handling. What power handling did the voicecoil have in the original woofer? It should be close.

Otherwise, an old *rough* rule of thumb is ~150watts per inch of coil diameter. ie a 2" coil is good for around 300watts.

 

just listen to it.... usually the low end will be mechanically limited.. you can turn it up at and listen for bottoming out.. or other stress it is obvious..
I feel that to be able to hear when your speakers or amp are distorting is critical.. Turn it down if it sounds bad! before you smell smoke.