To me this an obvious value move by Senn. Lets look at the HD660S, It's the same build but with a more expensive driver, using the one from the HD700. Besides being a more expensive driver but after the HD700 got discontinued, and it was still poor seller when it was available, its only the HD660s using it (with the HD800 and its kin using a different driver based on the ideas of the HD700). The $500 it originally sold for made sense. It was more expensive to produce even if a majority of the build was similar. But its still been the poorer seller and now is available at HD600 cost for the most part.

The HD660s2. It's in a family of $200-$300 headphones. It's using a recently reworked version of the typical HD600 family driver. So while there is some R&D to recoup. Its basically a HD600 tuned to be sound like a HD660s, with a bit more bass (compared to the S1).

If it was a full on replacement for the 660s and priced at the current going rate of the 660s or lets say a slight premium like $350. It might be a decent selection. A 660s with more HD600 characteristics (like the veil) and resolution. But at $600, you got 98% of sound with a $200-220 HD6XX, or you get a similar tune but with better resolution out of a $280-$300 HD660s. There is a debate of whether its general quality alone would in theory for a person looking for exactly it would be worth paying $600 for. But as a member of the HD6** family. It is an absolutely poor fit. The 660s has value for being a similar tune with a more proficient driver at half the cost and the rest for being so similar in drivers that the only major difference is tune and them being between half and a third of the cost.