I don't have the progressive tower but i bet they are similar. I haven't wired mine but have hung them in several locations to see what I like.

All 4 is not ideal on the front bar where the Roswell is. All 4 work on the back but i'm thinking I might put 2 on the front and 2 on the back. It is not ideal musically but worked best for my interior.

Yes the back bar is hollow. You can find several good post either here or on moomba about the best way to fish wiring and drill holes.

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