The Tyros design makes for LESS feedback possibilities. The speakers are facing the same way as the mic is - on up facing speakers, there is more chance.
This will not make me want a Tyros .... just making an acoustical point. If you balance the volume and "work the mic" correctly - you should be able to get plenty of gain from all the designs. My buddy that bought the XD9 has lots of feedback trouble when he uses the harmonizer ..... it's because it's like turning 3 additional mics on! Duh.
It's all about balance and moderation. Just like real life. Too much of anything is no good............except maybe redheads.