Ok, I've interrupted my normally scheduled listening on my AMP3 to check this out. I've only ever listened to the PK1's through the AMP3 (for this I'll use my Ipod Touch) so I don't really know what they sound like unamped so...
They actually sound pretty good. Very good in fact! Still with the punchy bass and nicely extended treble, generally good detail and everything is nicely balanced. Even at low volume I can still hear the kick of the bass, which doesn't always come through at low volumes using other phones. I'm using Metallica's Greatest Hits. I'm no great Metallica fan, but it's the only album on my Ipod. As I said, I use my AMP3 for audio. I need to say, that to have it at the low volume I have it at, and I do have it pretty low, I still have the volume indicator turned up to about 1/3 of the Ipod's full power. Also the sound is a little anemic without my AMP3. It still sounds good just not as full.
I can't comment on how the MX760's sound in comparison as I only have the PK1's and have never even heard the the Senns. but if you're wanting the PK1's, I see no reason why you shouldn't get them. They may be a little better with an amp, but it's not necessary and they still sound damn good without one!
Hope this helps. PM me if you have any other questions.
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