I have A Feliks Audio Espressivio MKII OTL amp. I'm not a great reviewer, but I can tell you that the 660S2 really shines with this amp. Clear, treble, great sound stage (compared to many SS amps), bass substantial and controlled. One of my favorite HP/Amp combinations. Hope this helps a little.
I like the s2 on a good solid state amp with power and control over the driver (Questyle Twelve for me). But my favorite is the Bottlehead Crack. I'm a sucker for natural tone and this combo exudes that tonality that I'm looking for.
I have found the S2 to be a lot pickier with regards to amplifiers than the 650 is. Some amps that are wonderful with the 650 (like my Woo Audio WA3 OTL tube amp) make the S2 sound way to bright for me. (I should mention here that I'm very treble-sensitive, so 'way too bright' for me may be just right for others.) On the other hand, the S2 sounds wonderful with the Monoprice Liquid Gold X, which I have found to be quite a temperamental amp which works well with some headphones but not at all with others. The Liquid Gold X - S2 combo is excellent.
I've tried the Schiit Modi, Magni stack which sound good for the price, I have also tried the JDS Atom+ Stack which I personally preferred slightly, and you can get both for similar price at about $200 They will both work well with the HD660s2, can't go wrong with either one, with great value.
FYI, JDS also came out with an upgrade in the last couple weeks, with the Atom 2 stack which offers now an all-around solid metal case and 2.65w per channel for $30 more each. This option will bring the price $60 but something that might interest you.
I'm actually using the "JDS Labs Element lll Mk2 Boosted with 2.6w with my HD660s2, This version will cost about $500 or $450 for B-class,
I have own and tried many amp/Dac combos recently from Matrix mini-i pro 4 Dac/Amp, Topping A90-D90 stack, and Hifiman EF400 Dac/Amp. I have really enjoyed them all, but pound for pound I really prefer and enjoy the "Element lll Mk2 Boosted" the most, especially with my HD660s.
I really enjoy how smooth and clear the music sounds, with a nice emphasis on the detail and nice clear thump on the low end, no matter how low I have the volume or how loud I listen to the music, the sound is constant. It also has an an onboard EQ that lets you adjust the Bass or Treble at whichever frequency you choose to raise it or lower it. It has all I need in one package.
I can't say enough good things about the pairing between the s2 and the Questyle m15 dongle. This is a PM to a friend from earlier today...(cross posted)
"Just wanted to share about my new dac/amp dongle. It's the Questyle m15 current mode unit. In this hobby I've often been pleased with amp/dac purchases, occasionally I've been surprised by the performance of a dac/amp but rarely have I truly been surprised at the performance of these miniature units. With the m15 I'm certainly surprised and somewhat amazed at the performance with this tiny but mighty pack of gum sized unit. I've only had it for two days, but IMHO it's giving the same performance/sound of a good mid range full size desktop amp. Heck, I'm getting between 85-90% of the performance of my $1500 Questyle Twelve desktop dac/amp. What? This thing is driving my hd650/660s2 effortlessly. Tone, control, staging, delicacy and nuance...it's all there. Works flawlessly on my Android phone and windows PC. You will have to hear this thing."
Pulled these out today and was enjoying them. After a few hours they were getting a little "thumpy", for my liking. I remembered that I had the iFi bass boost on level 2. The 109 Pro's needed that but these don't, IMO.
I pulled a pad off today after thinking about the @jonathan c mod with removing some foam. That foam is a little thick. Tomorrow I will do some A-B testing with and without the foam. I'll go with my test list, some new stuff and some jazz. Really looking forward to this!
Most of my headphones don't need or want level 2 bass boost, but the 109's do, for my preferences.
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This thing is driving my hd650/660s2 effortlessly. Tone, control, staging, delicacy and nuance...it's all there. Works flawlessly on my Android phone and windows PC. You will have to hear this thing."
