I have heard recently that SDX lamp from iwasaki recognised as have the highest quality of any sodium based light source. this means that this compares not only to another white SON lamps but also to regular golden orange high pressure sodium lamps and even to low pressure sodium lamps. because all white SON lamp provides the same colour light as the SDX although several suffers color shift. Whay the customers worldwide think that?

I guess experience. I have seen a lot of yellowed-out "white sodium", what in fact had "regular HPS" color.
 Philips state, then their lamps should be run only using their controller - i think it is because they suffer large thermal instabilities, so need an active arctube temperature control to maintain the color over the life. And (my guess) at the point, when the system reach regulator's limit, the colors tart to shift rapidly.
Iwasaki does not mention anything like this, so i assume EYE lamps have better made their pressure control, so could be run on regular unregulated ballast and still work well.