Diki,
all I can comment for now is that Ketron arrangers (like the SD1 or the Midjay) do have two drum tracks and this could lend support to your idea that they use some kind of REX loops (albeit Ketron calls them "Grooves" or grv. files).
Incidentally, the Midjay has a section called "DJ Loop" that contains pre-recorded loops that are really similar to styles, because they have an intro, four sections (A,B,C,D), a fill, a break and an ending. You can play them in sync with an external arranger (according to Ketron they work better with the SD5, where they can even replace the drum track). The best part is that Ketron has (on their website) a program that enables you to create your own DJ Loops starting from loops like the Acid; then you can import them into the Midjay, via USB, and use them together with an external style. The only drawback is that those DJ Loops don't follows tempo changes, and this means that the external arranger must be set at their original tempo. I have tried to play them together with the rhytm section of my Tyros 2 and the result is very interesting; even more interesting, however, is to play a Midjay style using only the drum tracks, because then you are not limited to a preset time. Moreover, this way you can merge the presence and liveliness of the Ketron drums with the beautiful Megavoices of the Tyros.

[This message has been edited by Dreamer (edited 06-29-2007).]