Real simply, SD1 is not a replacement for the X1. The SD1 is 76-note, no speakers--a newer model.
XD9 is more a replacement for X1. It uses the same case, but has new OS and sounds like the SD1. It has 61 notes and speakers. Vega is very similar but has ethnic features.
Previously Ketron keyboards were marketed by Solton. That's why the X1 says Solton. Now Solton is marketing the SG, but it is NOT made by Ketron, it is made by General Music and is virtually, if not exactly, the same as the GEM Odysey (I might buy one if I could remember how to spell it.)
I'm sure George Kaye or DanO will see this string and make corrections.
DonM