After a couple of months enjoying the HD660S2 with the Fiio K9 Pro (ESS), it was time to deliver my humble feedback to the thread
Introduction
To begin, I had a HD650 around 2016 and sold it with many regrets after maybe a year. Later on in 2020 was the Philips Fidelio X2HR which was just incredible for the price point (100€) driven by a Fiio K5 Pro (AKM). I had tried again the HD650 in 2022 but the bass was not so much to my liking anymore and returned it to the seller. Then came a home period since January 2024 for which I had time and money to spend on a totally new setup. I discovered the HD660S2 and it seemed to really meet my expectations.
The source I was the most interested was the K9 Pro (ESS). Alternative choices were the Asgard V3 multibit DAC and the DX7Pro Plus. Went for the Fiio and bought a XLR cable for the occasion. The idea was to get a very neutral source, all in one solution to avoid trying too many different DAC and Amps.
My thoughts
Until now, I have listened to a good variety of styles on QOBUZ - jazz - rock - classical - reggae - country - blues - electronic - and the 660S2 is clearly versatile. The signature is globally on the slightly warm side, still very natural. Treble is not so softened nor too present : for me it's the right balance. And for the bass, while it's not the most basshead headphone you will find, the impact can be visceral if needed.
And of course, I find back what I loved the most on the HD650 : those magical vocals and guitar/piano notes! I also feel more width, separation and overall technique to be on top. Of course, the K9 Pro helps to bring the best technical presentation possible. Compared to the K5 Pro, the power is of course more important, but the technical difference really shines on the well (re)mastered albums. It remains rather forgiving with poor recordings. On the sound ability, it is superior to the HD650 without a doubt.
Comfort wise, I have a bit smaller than average head and a thin face, hence what I like the HD6XX series for being so lightweight and kind of low profile. At first with the HD660S2, the clamp was lower than on my two previous HD650 which is a good thing. The low weight also allows for extended period without too much discomfort.
Conclusion
The HD660S2 paired with the K9 Pro (Ess) is a well balanced headphone, entertaining yet very detailed, and not fatiguing. Great comfort once the clamp is lowered a bit over time. Listening to well mastered albums is always a pleasure, while it can forgive quite well poor recordings. The best evolution I could hope for the HD650!