You are super generalizing here.
- You say you prefer closed backs and bass.
- You say the 660S2 sound clearer and better in the mids and better "just overall".
- You say you're "very specific about image quality" as a con for the 660S2 after saying that they are "clearer/better in the mids or just overall" (suggesting better imaging) while stating that you prefer to feel "isolated from the world" (while closed backs are known to have less accurate staging on average)...
So many contradictions. I'm confused? If you don't like them that's that. But your explanation counters your conclusion.
Yes but you shouldn't loose sleep over it. A lot of people have not branched out of one performance group, have noise/dirty power and sometimes have something specific to their hearing. I still remember when I got into audio and hearing how nothing made a difference, and then asking for someone to explain the difference between amplification types (D, A, A/B, etc) and basically no one could. Ironic.
Examples of what I mean. Amplification types that have an associated sound: OPAMP's, NFCA's, Tube amp's, discrete topology amp's, Forward Feedback amp's, THX, non-negative feedback amp's, Class A amp's, Class A/B amp's, Class D amps, current mode amps (Questyle), etc. Amp differences are easiest to spot based on the shape of the soundstage and how well sounds/effect/instruments are separated and layered. Amp performance is reasonably easy to gauge on an audition.
On DAC's I am usually surprised when people can't hear a difference in some. There are the obvious outliers like Chord DAC's or any FPGA DAC really (like Sony's TAZH1ES) and of course R2R or Multibit DACs, chip based DAC's are usually much more similar. The difference in DAC's is easiest to spot based on tonality. If you hear a cymbal does it sound glassy? If you hear clapping in an audience does it sound like clapping in a room or a clap track from 90's tv? Does a violin sound like an actual violin? Does a saxophone sound the same as a trumpet? DAC performance is hard to gauge on an audition alone.
I have no desire to change anyone's mind about any of this, but to answer your question, yeah, I mean they do often sound different if you're not only buying mid-tier and below and chi-fi or stuff from just one brand. Don't expect major differences though if nothing noteworthy in the topology changed (power, galvanic isolation, design, etc). There may be differences still but generally, I wouldn't be surprised to find that a D70 Octo sounds like a cleaner D30 Pro, but very little like a Pro iDSD Signature. Just what I have experienced first hand over the past 15 years since I got into headphones.