In the end, both the dx90 and x5 sound great, but I use them for different genres and different pairings to get the sound I like as well as for variety of signature. The key difference isn't that one is better, it is that they present the music differently. So what you are hearing from everyone in this thread is each individuals preference for the signature, and quite often, their preference in genres.
IMO, the X5 is intimate as if you are on stage in a small venue with the details right next to you thick with textures that you wouldn't hear at a distance. On the other hand, the dx90 is back a section where you can take in the music as a whole as if in a larger theater. The X5 is great for voice, electronic, etc that sounds great up close and the DX90 is great for orchestra, big band, live, etc where it is better to take in the performance as a whole. There are some crossover genres where the better match depends on the song such as rock or pop so these areas would be a toss up. To answer your specific questions:
- They are both neutral, but the X5 is warmer and thicker at the bottom, although both have powerful bass.
- The X5 can be played lower at volumes while achieving a full sound stage, but will get brighter as it is turned up to loud volumes. So if you like it loud, the X5 will show some brightness that bothers many. As a relatively low volume listener, this is not an issue for me.
- The X5 is the one for thick texturing given the right headphone pairing. Its my best match for my NT6pro for that reason. The DX90 presents its details differently with incredible layering for a bigger 3D sound stage.
- Different sound stages as describe above - X5 = intimate, DX90 = larger, live venue. However, the x5 with its thicker texturing feels more dynamic to me. Others would argue the converse.
- Both are capable of great detail, but presented differently, textures verse layering. This is more of a function of the HP though and its capabilities as you will see both scale nicely as you scale up your HP.
- Bass is powerful with both, but thicker more meaty texture on the X5 vs. more impact, faster on the DX90. For electronic, I always reach for my X5.
Given that you have a Amperior, that will be the signature that will shine through which I believe would be a v-sig so the x5 would help bring the mids closer if I am correct. Based on what you are saying, the X5 would probably be the better option in my opinion. But you would be very happy with either if you are upgrading from a phone/iDevice looking for another all in one.
oh and i forgot to ask one more thing,which is the maxium output power of the dx90 because i plan in the future to buy another pair of headphones till 100 ohm impedance and i would like to know if the dx is capable of driving them at their fullest without aditional amping.
