Choices, choices, choices... It's been almost 2 years since I was in your position, and I can tell you it hasn't gotten any easier in deciding which headphones to make an investment in next nor do I feel significantly more enlightened. Well, I could say I'm in your position again as I'm also looking for another pair of deep bass headphones, although I fear nothing exists beneath the sub $500 range. Anyway, back to your plight; you might have to make a leap of faith. As it turned out, I'm a basshead (though I didn't know that 2 years ago), and the first set of good cans I bought were the SRH440, and I will say they are the antithesis of a basshead. However, they were good headphones; deep down I realize that they brought out the true character of music in a way that the XB700 has not been able to do. The XB700 does a good job with frequencies upwards of 30 Hz, but the two headphones are fundamentally different: this is what we refer to as a headphone's "signature." Even if two headphones have similar frequency responses, similar drivers, and similar engineering, they will sound different (unless of course they are the same model). Even if you try to use all sorts of equalization or amplification, they will sound different.
Techno, electropop, and asian pop make up most of my music library, with minor representations from all other genres including jazz, classical, orchestral, r&b, hip-hop, rap, and so on. I do not think you will like the XB700 considering the type of instruments that comprise those songs, and if there is one distinct characteristic that the XB700 fails on, it is openness and ambiance (I'm not sure how new you are to hi-fi, but try to learn what the terms mean. It will make your life much easier). You would be surprised how a lack of openness or ambiance can ruin even a bass-heavy electropop song ("sekai turbine 2" by Susumu Hirasawa and "The Blue Monday E'R'Ectrical Parade" by Taku Iwasaki are good examples of this). The character of "sekai turbine 2" on the SRH440 is completely different than on the XB700; on the SRH440 it sounds playful and childish and on the XB700 it sounds more like I'm right next to a turbine in a dark, industrial city. But once you get past the impressive bass of the XB700, you realize that it doesn't make the song. Kukuk put it well; eventually you realize that bass is not all there is in a song. That being said, it is because the XB700 are such great cans that I realized this. I feel that they do handle real instruments well. The instrumental versions of "Hikari" and "Passion" sound just as impressive from the XB700 as from the SRH440, if not more. I also prefer "Tank!" on the XB700 than on the SRH440. However, the SRH440 does a more impressive job with vocals than the XB700, and "Mermaid Song" by Susumu Hirasawa has never evoked the same emotions with me coming from the XB700 as from the SRH440.
So it is not simply a matter of song type, two different headphones can play the same song differently and you'll like both versions equally well. Or maybe two songs in the same genre will sound better on one headphone than the other ("Inner Universe" by Yoko Kanno goes to the SRH440 while "Hemisophia" goes to the XB700) Closed headphones (XB700) and open headphones (HD555) sound fundamentally different [the SRH440 are closed headphones which sound very much like open headphones (but not completely like them)] Yet, some will prefer the sound of a closed back to an open back, or some will like both. Then, there are subtle differences in frequencies. Even though I call myself a basshead, I shun 40-120 Hz and I especially love frequencies below 20 Hz (~30 Hz is like the transition from hate to love). So to stop myself from rambling, it's really your personal preference. If you like it when your head rattles, then definitely go with the XB700. If you like it when the opera singer reaches the climactic note of the song, then the HD555 might be for you (although, I like "'Libera Me' From Hell" by Taku Iwasaki from the XB700 just as much from the SRH440). It is very rare that someone picks their perfect headphone on the first go, it is usually with the first headphone that you realize what kind of sound you like, and then learn how to get that sound (either by software (like DSPs and equalization) or by hardware (amps and DACs)). But that being said, headphones are only one part of the equation; without a good DAC AND amp, no headphone will sound good. Admittedly, I thought the guys hear overplayed the importance of DACs and amps, until I bought them and realized how much better my music sounded. That being said, the amp and DAC complement the headphone, not the other way around. Find a headphone you love, and then look for DACs and amps to make it better. But I would recommend you get the HD555. It'll let you experience the music and from there you can decided if you want the bass to go with it.
PS: If you get the XB700, keep the hair around your ears short or out of the way. The deliverance of sub-bass (20 Hz and below) depends on how good of a seal is around the cushion. If there is not a good seal, the air in the compression phase of the sound wave will be able to leak out and sub-bass notes will lose their power (and the head vibration they are known for). Also, low bass does not make the average human sick (high frequencies are also known to cause nausea in some people). Furthermore, there is no such thing as the lower limit to hearing. It is mostly a matter of amplitude, if the low note is loud enough, you will be able to hear it. Unfortunately, loud enough might mean decibels in the 80s or 90s, levels too high for most hardware to reproduce. You'll likely feel it before you hear it.
