You and I share a lot in common as far as things you are looking for in an all around headphone, type of music, source, etc... Much of my music is at 128kbps and I have to disagree with the common thinking that it's not worth listening to that quality on a good headphone. My first great headphone was an SR-60 and it made EVERYTHING sound better. So take what most of the folks around here say with a grain of salt. I'm not saying higher quality doesn't improve things, but it's not completely fruitless to listen to 128 quality music through good headphones, though it certainly wouldn't make sense to spend thousands on a great setup to listen to crappy quality music, but you're talking about dropping <$100 and I think you will be very happy with the upgrade.
Anyway, my favorite cans so far are the SR-80's, but for what you describe, I think you could get by with the denon d-1001's as a single headphone solution for awhile, then figure out exactly what you want before spending more money. The reasons for that recommendation:
1. Decent isolation, even at low volumes they will drown out ambient noise, people talking, computer fans, etc... Although they don't isolate GREAT for closed phones, they are better than any open headphone and don't leak much. IMO, they strike the perfect balance for portability (you can still be aware of your surroundings when you need to, like when walking outside etc..., but they can be used on a plane where you don't want leakage and there is a lot more ambient noise to address)
2. Light, circumaural, good padding, extremely comfortable
3. Bright highs without sibilance, good bass, slightly accentuated but not flabby or dark, just like you described. I enjoy the bass more than on the SR-80.
4. Easily driven without an amp (i use mine with my laptop and clip all the time)
5. Short cable with included cable extension gives you versatility for portable vs. home
6. easily modded (recable, dynamat, etc...) for potentially better sound.
7. <$100 used. I got mine quickly for $75 with a WTB in for forums. Don't feel like you have to go out and spend $200 just because that's your budget, give something less expensive a try and then you'll have room to grow. If you buy used, the depreciation is basically covered and it's easy to unload them with little loss if you don't like them or when you upgrade and outgrown them.
If I were you and had to pick one, this would be it. I've used or own everything that's been recommended in this thread. The AD-700's are great, but wouldn't satisfy you with their bass, and they are not headphones you'd want to wear in public, they are huge. SR-60's are awesome, but won't work portable or on the plane (long cord, poor isolation, leak sound), believe me, I've tried. I've never cared for the lower end Sennheiser sound, but that's just my opinion.
Anyway good luck finding what you're looking for, there are great options under $100, and an amp is not necessary for most of them.
JJ