I'm in the market for a new phone, mostly because my current phone can't consistently stream audio. Sometimes when playing a new track, it will only play 5 seconds then skip to the next track. This happens in Google Play Music, Pandora and Spotify. My current phone is a rooted HTC Amaze on T-mobile USA.
 
The plan is to stick to T-mobile until our family plan contract runs out in about a year, then switch to a carrier who has better coverage in our area. So ideally, I'd want a device that will work on AT&T or another carrier, as well as T-mobile. I've also noticed that the Spotify app stinks on Android, so I'm curious how well it does on iOS and/or Windows Phone. I've had a Samsung Galaxy S2 (T-mobile) in the past, and the sound quality on it was atrocious, so I'd like to avoid Samsung products.
 
Some of the products I've been considering are the Nokia 521 (which is dirt cheap right now) and the Moto G, which seems to get excellent overall reviews, but there's little info about the sound quality of either device. Please note that I am not eligible for a subsidized phone, so US$600 devices are out of the question. Budget is probably about $250 max. I know I'm not going to get audiophile quality at this price, but I would like to know what the best option is
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I plan to use the phone with my Sennheiser MX 365 earbuds, and occasionally with my HD280s. I listen to various forms of electronica (house, trance, trip-hop, ambient), jazz and the occasional blast of nostalgia (classic rock.) I seem to favor a bright sound to my music.
 
Any suggestions would be appreciated...

I wouldn't use Pandora or Spotify if you want the best quality, even their premium services aren't that good.. But to each their own.
 
But, the fact of the matter is that audio quality is pretty much an afterthought for phone manufacturers, especially on budget phones. The only phone that I believe was really marketed with "audiophile" sound was the LG G2, which is their flagship right now.
 
I would actually recommend a cheap player, such as a Sanza Clip+ for your music needs.

Your point about the audio quality of Pandora and Spotify is quite right. However, I use their free service to try out music that I don't feel like taking the risk of buying a CD by an artist I've never heard of. I also use them to listen to music that I like, but don't necessarily feel the urge to add to my collection. This gives me access to a far greater range of music than I could personally collect, despite the inferior quality.
 
The suggestion about a dedicated MP3 player has been mentioned to me. However, I'd want one that can also access streaming services, thus the interest in a phone. I suppose an iPod Touch might be a worthy contender, but I'm not sure what kind of audio quality to expect from it.
 
I'll look into the G2, but the price is probably too much for me. Of the budget phones, has anyone run across one with at least halfway decent audio quality?