Just ordered a pair of refurbished HD560S to see if I can detect the driver DNA between it and N**-3* and the 660S2
looking forward to your impressions! i have the S2 and 560S, but not the n30. Between the S2 and 560 they have a kind of house sound similarity, but are very different up close. Sennheiser's house sound IMO is one of overall balance with strong mids and a neutral to neutral-warm tonality, with the top and bottom frequencies varying the most from model to model. I feel that does apply to the S2 and 560 on a macro level. But as you dig deeper they're very different sounding.
The bass on the S2 goes deeper and is of much higher quality, IMO. The bass on the 560 seems a bit one note at times. It's either bass on, or bass off. That's oversimplifying of course, there is nuance there, but compared to the S2 that's how I hear it more or less. The S2 has more bass control, texture, and resolution.
Both have good mids but the 560 is more in line with the older 600/650 in that it pushes the mids forward a bit, whereas the S2's mids sit more evenly between the bass and treble. Tonally the mids are cooler sounding on the 560, but more forward and present. The S2 again has the edge in resolution, control, and overall technical ability.
Treble IMO is very different. Both have excellent extension and presence, but the S2's treble is of higher quality. I find the 560 slightly fatiguing for long sessions, with a treble response that is harder and more tiresome. The S2's treble is smooth and trouble free for me, and i'm very treble sensitive.
Soundstage is the one area the 560 excels at vs the S2. It's wider and gives more of a sense of space. The S2 is more classic 6 series intimate. Not bad at all, but not much sense of space and scale. What space there is though is well layered and separated.
I love the 560 overall and think in it's price range it's tough to beat. But the S2 just plays in a higher league IMO. Would love to hear other's impressions.