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You're using your computer's onboard audio as the sole source. Consider this is probably going to be the weak point in your chain. Make sure your music quality is highest as possible.
At your price point, asking for a detachable cable means you haven't even looked at what's available. Detachable cable is not something that comes with premium headphones. Accept it, or be prepared to deal with the limited selection.
I would say look into:
Mr Speakers Mad Dog + Alpha Pads
Beyer DT880 32ohm
Beyer DT770 LE 32ohm
Ultrasone HFI 2400
And at the very least, look into even a used Fiio E10.
I had a M100 and I really loved its sound signature. First thing you would notice is the bass-emphasis! The bass is really tight and accurate without bleeding into the mids/treble. soundstage is great for a closed pair of headphones, but for closed headphones, soundstage will never be as great as open ones I think. That's just the trade-off you make for noise isolation.
The M100 will definitely have a good solid seal. Maybe even too tight the first time you try them on, but the flex-headband will adjust very well.
One thing I would caution you about with the M100 is that I had comfort issues d/t the inside of the earcup driver pressing against my ears and I consider my ears to be small, but I am really sensitive though. V-moda (60-day) and amazon (30-day) both have great hassle-free return policies though, so I would still recommend you try them. One way to fix this is by stuffing paper towels under the earpads and V-Moda says they are releasing XL earpads soon. Hard to say if you will experience this same issue until you try them, so I think you should go for it!!
The Pro 900 would fit your requirements. I would suggest you consider them in comparison to the m100s since the m100s are fairly new and still have a street price near the list price. The Pro 900's, on the other hand, have been around for a while, and are reaching an all time low price. This makes me think the m100s will be worth less in the future and end up being discounted like the other V-Modas, which the Pro 900 should be more resistant to.
I don't hear pro900, but pro 750 sounds excellent
SoundMAGIC HP100, or the new ADL H-118 looks promising, but it's not been released yet.
Val said prototypes of the XL cushions will be released on March 14th! Subscribe to this thread for V-Moda M100 updates: http://www.head-fi.org/t/592389/v-moda-m-100-discussion-feedback-reviews-pics-etc/12690#post_9231526
If you really like your ultrasone HFI-580, then an upgrade to ultrasone pro 900 would make a lot of sense. There are a baller lightening amazon warehouse deal for the ultrasone pro 900 at $200 a few days ago and a pretty sick coupon code for $250 M100 from Guitar Center that just expired. I don't know how urgent or how picky you are about your final headphone choice, but sometimes you can find real bargains by subscribing to the deals threads here. All headphones lose value over time depending on their companies product refresh cycle, so if you find a sound signature you really like I wouldn't worry to much about the price.
When you say that you are worried about cable failure or product failure, one thing that I really like about V-Moda company is its customer service. I ran into a cable issue with the V-Moda M100 and they legit spent me a new one without blinking. If anything goes wrong with your headphones within the 2 year warranty period, I am sure they will replace them without a problem. I am sure that Ultrasone has to have a good customer service/warranty as well. I really think with a lot of higher-end headphone companies, they will treat you very well if you are spending money on them.
I find a good approach is to have a list of headphones that might work for you in mind. When they pop up on the deals thread for really low prices, go for them. You can usually sell them off and not lose out if you do that. That way you get to try more headphones and find ones that you actually like. Reviews give you an idea of what a headphone's like, but ultimately you're the only one who can tell if a headphone is good for you.
did you do a direct comparison??? how was the bass compare? I really loved the M100s bass (seems really textured? not sure if that is the right word though).
could you do a more detailed comparison pleasey??
I haven't heard either, but if I was looking for $300 closed cans and portability wasn't a must-have quality, I'd choose Mad Dogs any day. I got to see the alpha pads on a youtube interview and they look so comfortable....