I can’t comment on Spotify or other streaming service, since there no way to verify the sound quality. If I have CD or sound file, I can at least verify the numbers.

Talking about numbers and formats, I can’t tell difference between MP3 192kbit and FLAC (source file).

For Mid-Fi:

• Source: YAAR!

• DAC: 5%

• AMP: 20%

• Headphones: 75%

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If we talk about dedicated solid state headphone amps with similar specs, the difference might be difficult to tell apart.

If we compare poor laptop PC audio and dedicated headphones amp, the difference might be in noise level, volume and dynamics.

For open back headphones, I’d consider Sennheiser HD599 or HD560s. Just make sure to understand merits and limitations of open back.

What it is your current setup?

That would be a crime considering that HD660s2 is like 2 times more expensive.

But we can’t really tell, unless someone with knowledge open both headphones open and investigates it.