HD555->ATH-AD700?

No, save your money for later upgrades. Also turn it into a 595. I haven't heard the 555 but I've heard the 595, and it is on a similar level to the AD700, but I wouldn't get both because by saving what you have and selling what you have, you can get a mid fi phone.

From what I've read, they are certainly better phones. (And I have them, they are certainly very good for the price)

What Trysaeder is saying that it would make more sense to sell the hd555 and buy a more expensive headphone.

Perhaps, DT880 or HD600's

But either of those need an amp and would benefit from a better DAC than the one you have.

so, it really depends.

+1 for saving your money.

I've heard the AD700, and there is little, emphasis on the little, difference between these two phones. Your money would be better spent on a decent amplifier, (cMoy, Fiio, Headroom), than a new set of sub 100 dollar headphones. From my experience, the HD 555's always sound better with an amp. Almost definitely.

Perhaps a better investment would be the HD 600 or 650's?

I have both and I use them solely for gaming however occasionally Ill use them for late night movies. The AD700s are great, excellent soundstage and the positional cues are outstanding. I just recently acquired the 555s and so far they seem just as good. I havent had a chance to mod them yet to see if there are any improvements but so far my only gripe would have to be with how they fit. Apparently I have a huge noggin and they are a bit snug. Performance wise I think they are pretty much on par however I do say the AD700s have a bit more of a soundstage. Personally I prefer the HA-RX900s over my AD700s but Im going to start using the 555s more.

Im using an Astro Mixamp to push them now and I will soon have a Bravo amp to accompany it so I expect some increases in performance from all of my cans.

Honestly for the price you can get them for not to mention the potential resale value they have, I would say go for both if your budget allows it. You can always sell them and use the money for something better but give it time.

Technically your sound card is you amplifier so if you used an amp with it you'd get pretty nasty sound (Amplifier Piggyback Effect). do you already have a good sound card? If it's a crappy sound card then you are better off with a portable USB DAC/AMP like he Fiio E7, one of my favorites. Also, the ATH-AD700s are nice, but I own the ATH-A700s (Closed Back Version) and they are much better IMHO. Both are much better than Senns. Senns are usually "veiled" sounding, or "colored" and in my experience, the Audio Technicas are not.

Hmm. That looks like a fine soundcard to me, based on the specs. Upgrading the Opamps may or may not have a slightly noticable difference. You may or may not be able to tell the difference.

Since your original question was about switching headphones, I would have to answer that many people swear that AD700s are better than HD555s, and almost nobody says the opposite. The decision is yours. I own the closed back version of the AD700s (Mine are the ATH-A700s) and I love them. They are my first audiophile headphones, but they are still awesome. I just took them to Florida and I couldn't have survived the plane ride without them.