I'm currently using an SLI-80 with the Grado HP-1. A truly magical combination, at this moment. I've rolled the tubes a little though, so my Cary is not exactly stock. (Svetlana KT-88s, and Amperex JAN 7308 Orange labels.)
I've not compared the SLI-80 with the 300SEI. However, fellow head-fier Carlo has. He likes the SLI-80 a little better. You can search for a recent statement to that effect. I think I remember him rating the "80" a 9.5, and the "300" an 8 or 8.5, out of 10.
Hope this helps.