[squelchy451]

Hi. My current setup is Sennheiser HD428 (recently developed an intermittent buzzing noise, so want to buy a new one) and Sansa Clip +.
 
I listen to mostly classical music.
So far, headphones that I might buy are ATH AD900 (if I can find them used), ATH AD700, Sennheiser 558, maybe Audio-Technica M50, but I am looking more for open end headphones, as I usually use IEM at the library.
 
I'm looking for something that reproduces classical music as performed in a concert hall, hence the preference for open-end headphones.
 
Below $150, what are my options? Amps, DAC, etc are not needed at this price range, correct?
 
Thank you

 

[Spastic]

In terms of classical music, I would recommend the Sennheiser HD558's, not so much the M50's since they're more for the faster paced genres such as pop, rock, trance and tend to be bass heavy (imo). 
 
Sennheiser has a very open and airy feel to their headphones and are fairly soft on the higher frequencies. AT however in comparison are known to be bright and some people don't like the sparkly treble. 
 
I think a 558 would suit your needs and to perform the foam mod on them would make them sound even better. 

 

[PurpleAngel]

I own theHD558s (removed the black rubber tape) and I do like them as good jack of all trades, but as the HD558 comes with a 1/4" headphone jack, you have to use a 1/4" F to 1/8" M adapter to plug them into a portable (Clip +).
The Shure 1/4 to 1/8 (6 inch long) cable adapter would be great to use with the HD558s.
 
But I'm going to guess the ATH-AD900s might be better music headphone,
as the HD558 can come across as a little "cold", some say "veiled" for music audio
(I think Sennheiser expects the HD558 to be plugged into receivers, as receivers seem to pump a lot of juice into headphones)

 

[Spastic]

1/4" headphone jack, you have to use a 1/4" F to 1/8" M adapter to plug them into a portable (Clip +).

Yeah true, you end up with a pretty large plug that you have to insert into any device that accepts only the 3.5mm jack.
 
I haven't tried the AD900 so I can't really say anything about those. I also forward the notion that the Sennheiser's might sound veiled to some people, for me the HD598's are strangely veiled but the 558's are not.