Hi Houman-
I'd say that the M5 v2 would be a much better choice than the Custom 3. The Custom 3 had no bass to me whatsoever, but I could never get them to fit right (even when holding them in place they still had no impact). The Atrios have wonderful bass. I think you'll find their bass much more preferrable over the C551. Between the v1 & v2 Atrio- the v2 retains all the bass goodness of v1 but has improved treble and mids. Also, build is better since they removed the memory wire, and beefed up the cable. They're quite the bargain. I'd stay away from the Custom 3, even at half price.
I was reading a few of your posts and saw the list of phones you have been using and was wondering if you can help me out with a question. I had the Sennheiser CX-300s and then the Denon AH-C551. I liked the bass on the CX-300s but the AH-C551 was tool flat for me. I’ve been looking at Future Sonics Atrio m5 and Klipsch Custom 3. It seems you are familiar with both. First question: for the Atrio M5, what is the difference between V1 and V2. Question number 2: Considering that I definitely appreciate balance and detail in all frequency, however would certainly prefer more lower end to the high details if there was to be a trade off, would you recommend the M5 or the Custom 3? What is all the fuss about Westone 3?