The alternatives to the a6 arent numerous as its one of the few modern analogue polys with a keyboard. The other would be the dsi polyevolver keyboard, although its only 4 note poly and a ana/digi hybrid.
Studio electronics make the omega 8 but its a module only.
From the old guard it has to be a jupiter 8, prophet 5, or memorymoog
BTW i dont believe that any poly can sound as good as the memorymoog. It will easily destroy the a6 in a head to head!