Awesome. Well, I don't have any experience with the 400, but I can say I'm very pleased with the 400i. It's a gorgeous headphone, however this has led me to some obsessive-compulsive cleaning as the gloss finish shows fingerprints and dust quickly. They are comfortable, but I find the headband adjustment a little quirky. The X2's are definitely more comfortable in my opinion. One of the most comfortable set of cans I've worn. Soundwise, the 400i is more neutral and precise, with the mids definitely stealing the show. As I'm sure you're aware these are not bass-heavy cans. The bass is there when the recording calls for it and it is tight and controlled, however overall I find myself wishing there was just a little bit more oomph. Some may disagree; if you like a balanced sound, these are superb. However if you listen to a lot of electronic music you'll probably be underwhelmed with these, although playing them through a good amp/dac does bring out the bottom end. That being said, the 400i's sound awesome. Less spacious than the X2, but more detailed, present, and responsive. Again, the real magic is in the mids and highs. The mids are lush and fluid and the highs sparkle without being harsh. Instrument separation and texture is amazing. I would probably rate these phones as low high-fi, where I'd probably place the Philips in the mid to high med-fi. I've heard them called a little cold and analytical, but I disagree. They are very musical and enjoyable. A bit more picky about amplification than the X2; I'd definitely put some good equipment behind them before judging them. I usually play them through the Musical Paradise MP 301 MK3, which is a decent tube amp, and I can say that vocal-driven music, jazz, and rock just sing through these. Adele, Springsteen, Velvet Underground, Pink Floyd- I literally get goosebumps. My overall opinion: a stellar sounding set of cans, but a little picky. If I had to choose one single pair of phones to own, these would probably not be it. However where they shine, they really, really do shine.
The X2, on the other hand, is a killer headphone in its own right. I may be somewhat biased, but I love these phones. For the price, I don't think you can beat them. They don't really do one thing exceptionally, but they do everything well. They are comfortable, look great, and are just fun. I own much more expensive pairs of headphones and still find myself on these most of the time. The soundstage is airy and there is distinct instrument separation. Bass is incredible for an open-back headphone, especially sub-bass. The bass doesn't quite have the quality of the 400i, but the quantity is there without being overwhelming. It does bleed a little into the low midrange, but they're so enjoyable to listen to that it's not a concern to me. Has a bit of a V-shaped signature yet the midrange is still pleasingly present. To me the sound is in the same vein as the HD 650- not quite as detailed but more lively and fuller bass; given the choice I would take these over the 650. Also not nearly as detailed or textured as the 400i, but very warm, musical, and non-fatiguing. You can listen to these all day, and they sound good with whatever equipment or genre of music you throw at them. Granted, they're not going to rival flagship headphones in sonic accuracy, but they are an all-around enjoyable workhorse, and a steal for what you pay. For a solid, rocking med-fi headphone, I would recommend these all day.
