jw54

I am looking to add a 15" extension cab to my Carlsbro Viper 250 combo but there is not speaker output. The outputs I have are: DI (1/4 " jack on front) Line out (Active) send & return Bi-amp Hi out & Lo out with frequency selector (200, 400, 1.5K & 2K) Can I connect the speaker to any of these? There is no indication what the impedence of the internal 15" Eminence speaker is. Thanks for any help offered.