upgradeable dacs

I am tired of wasting money on buying dacs to have them be usurped by new technology within a year of purchase. many dacs that claimed being upgradable either were not, or were so expensive as to be not worth it. (original berkeley and bel canto come to mind) Other than the new nad dac line which i think really can be upgraded, does anyone know of higher quality dac products which can be upgraded without any nonsense? thanks