The 515s are, especially compared with other sennheisers, pretty good value for money.
You still have to like the pushy, bassy sound sennheisers produce.
I think most sennheisers are very tiring to listen to.
The only one I've liked so far was the cheap ass Manhattan. That's actually pretty good
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Using an adapter should mean a decrease in sound quality, but I can't say I really hear it. I wouldn't be too bothered about that. I must say though that most more expensive headphones already have an 1/4" plug. There's also adapters that go from 1/4 to 1/8"