The iPhone and the iPad don't (and a lot of other playback devices) like to have their Left and Right outputs shorted together. A 3.5mm to 2- 1/4" connectors will usually work. or you could buy a cheap DI type of device like the Rolls DB14 A/V presenter http://www.rolls.com/product.php?pid=DB14 it will combine the outputs of the playback device and give you one XLR output. The only issue is it has a gradual roll-off from about 150hz down.
The spec on these may say 150hz roll off but having measured at least 10 of them I can tell you it starts at about 1K and is not subtle. I was wondering why the music at the schools I look after had no bass but the microphones sounded fine- The problem was the Rolls DB14 boxes they had been buying for years. I can't believe a product would be on the market for so long and sound so bad.
Rapco's LTI BLOX is my preferred simple, economical balancing/summing interface box now.
I played around with a few of the Rolls DB14 trying see if it was something simple that was miswired inside but it seems that the transformer used is wrong for the application and the HPF is inherent to the design. When I find them now they usually end up in the E Waste bin.
