Respectfully, techno_dude,... I do not agree with your comment. The Ref 6 is a wonderful preamp. The SE upgrade made the Reg 6 sound better. I understand that the changes consisted mostly of new caps and wire.
ARC is constantly trying to improve its product line. If upgrades to one component can be transferred to another product, ARC oftentimes will do that. For example, ARC made SE improved its Ref 150 amp, the Ref 6 preamp and the Ref Phono 3.
I own the Ref 150. ARC did the SE upgrade several years ago. The upgrade included changing out the KT-120 tubes for KT-150 tubes, plus other changes. I can tell you that the SE upgrade significantly improved the SQ of my Ref 150. Ditto re the Ref 6, which I used to own but had upgraded to the SE version.
Anyway, that is my opinion.