I bought an LG with replaceable battery and expandable memory c. 2018, and won’t downgrade to a phone without those two required features. It talks, texts, browses, navigates, and plays music from a headphone jack. That’s all I need.

550mW into what resistance? I wish marketers would understand that the number is meaningless without context. $220. You could just buy a K7 and a $20 keyboard, and have much better sound. Although the volume knob on the keyboard is nice.

550mW into what resistance?

I have both the 6XX and the 660S2. The 660S2 has far better imaging and bass. The 3D layering is fantastic. The bass is quick and deep, textured, detailed, and revealing. On Beck’s “Loser” I always heard a slurring bass line on my 6XX. On the 660S2, I can make out the individual notes. The lead vocals on the 6XX stand out beautifully from the rest of the music, which tends to blend together into one of 3 blobs. On the 660S2, each and every voice and instrument is completely separated from the rest, and the circle of sound around your head is complete and accurate.

The 6XX is more relaxing, more beautiful, with slightly better mids. It is more natural and more neutral. The 660S2 is more V-shaped and energetic. They’re both great with classical, jazz, and classic rock. The 660S2 adds pop music to that. Neither is ideal for hip-hop or edm, or anything that requires a bass cannon that can throw 20Hz hits at you. The 660S2 is reliably good down to 30Hz. Stand up bass viols sound natural, not like the lowest notes are accentuated. The 6XX is reliably good down to 40-50Hz.

I use the 6XX for quiet background music while I’m working or on reddit. I use the 660S2 for critical listening. The 6XX is never quite breath-taking, whereas the 660S2 sometimes is-- even at low volume. The 6XX is always makes comfy sounds. The 660S2 can be distractingly good.

I buy new pads for my Senns every January. There’s always an improvement in bass.

They are good because they are dark and subdued.

I disagree about the line being easy to power. They’re easy to power competently, but not easy to power well. They definitely benefit from having a ton of power at their disposal. The 6XX with over 500mW available has improved bass extension, better dynamics and microdynamics, less of the 3-blob problem (better imaging), and better detail across the spectrum. I can switch between two excellent dac / amps (the MCTH and the K7), and hear a lot of differences. The MCTH makes them sound even more like what they sound like: smooth, natural, with blurred images (except the lead singer, who always stands out as if by magic) and thick but shallow bass. The K7 tightens up the whole picture, adds more sub-bass, greatly improves imaging, and if it doesn’t quite make them sound quick, it at least makes them sound less sluggish.

Try them with a Fiio K7. I’ve tried my 6XX with amps and dacs from Schiit, Cavalli / Drop, and Topping, and the K7 brings out the best in them. $200 for a dac/ amp, and you will never need another.

Yeah, that is a problem. More of us who have been around for a few years need to hang out in the stickies.

Yeah, that’s all right.

The HD660S2 is a great deal for under $500. It’s my favorite of all the headphones I’ve ever tried. I have tried to EQ it, because it does have a low-mid hump and a peak around 8600Hz that I had wanted to smooth out. They’re not offensive together. The one balances out the other. But it would be better if neither were there, in my opinion. But getting rid of them backs the stage away and puts the singer and band behind a veil, due to the huge dip around 4400Hz. That dip cannot be lifted without ruining the clarity, which is one of the main selling points of this headphone. I have given up on trying to EQ them.

The bass is probably the other great selling point. It is low, agile, quick, and beautiful without being overbearing. It’s the best bass I’ve heard on a dynamic headphone. With it, I have learned that there are more notes in the bass line to Beck’s “Loser” than I had previously heard. It’s the opposite of muddy bass. I tried giving a tiny boost to the lowest frequencies just for hip-hop purposes, but it also ruined the clarity. I just can’t use any boosts in EQing them, at all.

So, to sum up: They’re great headphones. I use them every day, and I love them. They have shown me new sounds in old songs. They fit and are built very well. They have excellent, natural tonality in general. But they have that treble peak, and the low-mid hump to balance it out and give you a V-shape. If I am using the K7 with the 660S2 without EQ, the treble gets fatiguing within a few hours. If I use the MCTH with the 660S2 instead, without EQ, the treble peak isn’t as cutting. I can go several hours with it. The sound is warmer, more rounded and organic, but with less punch and not as well-defined layers of depth. If I use them with either amp and EQ to reduce the treble peak and low-mid hump, the tonality is perfect, but distant and dull.

So even though you’re breaking rule #2, I’m going to advise you get them and see how they do with your amp. If your amp is technical, analytical, and powerful, that treble might annoy you. If your amp is warm, organic, and rich, you’ll probably love the 660S2 with it.

My first audiophile headphone, and still the one I always come back to. There is simply no better headphone for playing music quietly in the background while working. It’s relaxing, natural, beautiful. It never wows or distracts, but it’s never quite boring, either. It’s far too musical for that. It has a very unique sort of magic. It does benefit from excessive amp power.

A fool and his Hifimans are soon parted.