The AX were amongst the first affordable PCM sampled keyboards, at the time when Yamaha were still using low operator FM synthesis as their sound generators, and the sounds were a huge step up in quality in relative terms at that time.
The AC1200 was just a KN1200 in a box without keyboard or speakers and was quite popular.
The WSA1 was an attempt to break into the synthesiser market with an interesting acoustic modelling approach. It still has some followers today but mainly gave us the trackball on the 6000/6500.
The Panasonic KC were entry level "Christmas Present" type affordable keyboards to get kids interested in music.
The numbering is quite logical since the 1600 is a mid range board, successor to the 1500, which was a successor to the 901. In the same way the 930 succeeded the 920 which succeeded the 701.
The 1600 was five years after the 2000, which was a top model, so a different series of numbering, and although by then it had caught up with a lot of features, and overtaken some, the 2k still had the solid construction and better overall speaker system of the top models.