how to fix modified square wave AC

The power conditioning options are much more extensive in the audiophile world than in the music production world, so I am seeking your assistance. I am trying to put together a portable rig that would include a couple of keyboard-synthesizers (Kurzweil K2661 and Yamaha Motif SX6) and a pair of Motion Sound 100-watt keyboard amps. I purchased some Xantrex PowerPack 600HD batteries, and discovered that when I plug in the amps, they emit an obnoxious buzz. Neither a Furman Elite 15 Power Filter nor a Furman IT-1210 Balanced Power Transformer quelled the buzz. In fact, the Furmans themselves began to buzz when I plugged them into the batteries. I suspect that the problem is that the inverters on the batteries produce modified square wave AC and my gear needs pure sine wave AC. So my question is, What power conditioner would work best for my purposes? The PS Audio Power Plant Premier seems like it would take care of the problem, but I would prefer to purchase a lighter weight and less costly power conditioner – if it could do what I need it to do. Would a PS Audio UPC-200, a PS Audio Duet, or a Magic Audio Stealth Mini Reference do the trick? Would I be better off with the Magic Audio Stealth XXX (even though it costs a bit more)? Or would something else be more suitable? Any assistance would be greatly appreciated. Thanx!