I use 2...it just sums the input anyways...you can use a little less gain.
But you don't need a y---just use your sub out on your pioneer (i have the same one) and just plug it in either the left or right port on the sub.
But you don't need a y---just use your sub out on your pioneer (i have the same one) and just plug it in either the left or right port on the sub.