I posted a short review of the Cayin 15 vs the 17, as I have both.
agree with Audiosu. The 17 gives more complexity to the music, though as I said in my review, the 15 is a very good unit.
My next move is to get 4 upgrade tubes , Siemens. Might even make more of an improvement over the 4 russian tubes.
For a $100, its worth the chance.