i have only overheard a bloke demo them in my local Richer Sounds and the salesman said that the 650a had slighty more control at the bottom end(i thought the upper bass was close but the 550a sound a little boomier. But the 650 i found was very bright when he turned it up maybe it can be said that top end has more sparkle. I would say if you really want one i go for the 650 as it did seem to more controlled when the bloke had some pan pipes music on. I think he has some MS alumini sats on very odd, so not any really sounded polished.