I got my quads yesterday & they're heavenly. They are my first set of customs too.. and I couldn't be happier. You can see my gear to get an idea of what kind of sound I like from my IEMs (balanced, with a premium and detail and clarity).
In terms of the overall package, the quads surpass all the other IEMs I have.. easily. The RE0 comes close in terms of microdetail retrieval, but the quads simply blow them away when you take into account depth, timbre, and weight. The quality and quantity of the bass from the quad is absolutely unreal. I've never heard bass sound so effortless, textured, deep, rich, and powerful. The really impressive thing is that it only flexes it's muscle when called on.. so everything you listen to won't have a bassy tinge to the sound. The midrange is ever so slightly forward and the treble is smooth, but boasts copious amonts of clarity and excellent crispness & air. It's a smoother sounding phone, so I can listen to these for hours on end without fatigue.. and that smoothness in no way is a detriment to enjoying the microdetails in all your music. As I said earlier, a RE0 amped well comes close but the quad still reigns supreme and has a more rounded note and timbre. Microdetails retrieval is simply one part of assessing the technical and musical ability of a phone.
There seems to be a good amount of competition in the 400-600 range for customs.. when you're spending this much on phones, I'm willing to speculate it's less about what phone is "better" and more about finding a phone that suits your taste. I'd think the technical differences between customs in this range are much closer than or I might think.
Understanding what kind of audio characteristics you value and what kind of music you listen to will make the search much easier.
I was a little concerned the quad would be too bassy & warm for my taste since I like a more balanced, detailed, crisp sound.. and the quads did not disappoint.
I'm not actually partial to a particular sound.. I just like phones with good balance, a cohesive presentation, and have great VFM...which the quads most certainly are, IMO.