One of the biggest issues on the X1 (and there are only two real issues), is that the ear pads aren't removable. This makes it quite problematic to clean or replace. It is held in by four screw-like protrusions, and a strong adhesive. This makes it a quite a commitment to remove and place back on. Considering the masterful design of the X1, the ear pad assembly comes off as archaic and incredibly cumbersome. You may have to contact Philips for a replacement (possibly for a price) once the pads start wearing out. Problem there being that Philips quite possiby won't be in the headphone business soon, so there may not be a legitimate method of replacing the pads. You will definitely want to baby the pads, and try to keep them as clean as possible at all times. Some tape to remove particles/dust/etc, and not using the X1 when you're dirty.
The second (though less problematic) issue, is that the stock cable (as amazing as it looks and feels) has a very high resistance (around 1.8ohm). That is unnaturally high for an audio cable, and it does cause a negative effect to the sound quality. Compared to audio cables with a typical resistance of around 0.5ohm, the stock cable makes the sound slightly congested, and slightly undetailed, which makes instruments sound a little hazy and blended into the background. Replacing the cable will immediately tighten up the bass, and better define instruments, and other sound effects clearly in comparison. The difference isn't vast, but it is there, and can be noticeable with the right material. Replacing the cable isn't costly, and you can replace it with something equally sturdy such as the Mediabridge audio cables sold on Amazon for $10 or less for an immediate improvement. The only positive aspect of the stock cable's sound is that it's warmer, and less fatiguing, due to the softer, less defined sound.
The third issue with the X1 is that the headband simply isn't made for larger heads. The space between the suspended headband padding and the leather covered top that connects the cups is quite small, and once you put the headphone on, the suspended headbasnd will crash into the top piece, not allowing clearance for larger heads. There is a simple solution to this, and that is to bend the top piece into more of a cone shape, to allow more clearance. There is a lot of wasted horizontal space by default so bending the headband allows this unused space to be occupied by the suspended headband if needed. The top band is all metal and leather, and won't break, so there shouldn't be any worry about damaging the headphone with this mod.
