I have the 660S2 and the K9 Pro since a couple of weeks, it's just fantastic! They make a great combinaison.
The K9 Pro is big step coming from a K5 Pro (AKM) as well.
in the past, I had the HD650 (then sold it) and wanted to have it back, but in a more modern formula. When I discovered the 660S2 I jumped on board, although the price (470€) was not so friendly. I also had tried the 109 Pro, but wasn't appealed by its treble presentation at first sight.
Together, the K9 Pro (ESS version) and the 660S2 provide a very nice balance, the neutral approach of the Fiio and the Sennheiser HD6** warmth