Bob,
When the SD1 came out almost everyone I sold for several months were upgrades from the X1. The comments from all the X1 owners, except those needing middle eastern drums and styles, thought the SD1, just for the new Piano sounds and easier to use interface was worth the trade up. Now, almost 2 years later, those who moved up to the SD1 love it for it's sounds, extra keys, the ability to play styles direct from disk and especially how much more convenient it is to choose styles and sounds from a much improved operating system.
There are many good choices for an external sounds system. Many Ketron players have been using the very light and inexpensive Yamaha YSTMS50 80 watt system which is around $100 dollars. The Yamaha MSP5's are good, but the Midiman Studiophile 5's are 50 watt biamped and approx. the same size as the Yamaha's for $299.00 where the Yamaha's are closer to $450.00 for the pair.
Let me know if there's anything else you want to know.
George Kaye
Kaye's Music Scene
Reseda, California
818-881-5566