John
I forgot you didn"t use a mixer.Here is a good one http://www.musiciansfriend.com/srs7/g=home/search/detail/base_pid/630156/
,or a Y cable from your mono out might work with a powered sub.You would have to consider relative volume,however,and a mixer might be better suited for the task.

The best choice IMHO is a Bose PAS. I took the plunge, and am confident in having the best vocal and instrument sound available at any price.