Summing Balanced Inputs to Balanced Mono Output

[dwmoran]

I've got a stereo L,R pair of line-level, balanced (TRS 1/4) outputs coming out of the back of my monitor section that I'd like to sum to mono to go into a JBL EON Sub, which takes balanced mono inputs. I'm out of balanced mono outputs, so I can't just stick a cable into an L=mono configuration and go. While I've got it breadboarded into the EON at present, the way I've done it promises phase/impedance matching problems on some material.

Does anyone know of a box (active or passive) that will sum two balanced stereo inputs to mono and then provide a balanced out? I don't need to mix the signals -- just sum them.

Thanks!

Don Moran