I have been going to & fro the Quads and my ES5 for most music I have been listening to for the past week.
Ultimately, to me, there is no clear winner.
Wearing two hats as a sound engineer and semi-pro bass player, I would pick both for different purposes.
As a sound engineer, the ES5s are my clear choice given their flatter response and better treble extension. I find I can come to a fairly close mix while using the ES5s.
As a bass player, I used the Quads briefly as monitors for a gig I sat in during the weekend and they absolutely rocked! There was enough low end punch while keeping the definition. The ES5s do the same job but lack the low end punch that's needed for live "energy". The Quads would definitely be my go to IEMs in this case.
Listening to music for work (mixing, analyzing and transcribing), again, the ES5s are my choice, having a very wide & flat frequency response, good separation and sound stage. I have to say, contrary to popular belief, the ES5s DO NOT lack bass, it's just not boosted like most other headphones. It's there when it's there in the recording.
When listening to music "for fun", I go to the Quads for rock, alternative/pop, "LIVE" albums and bass heavy music. The Quads really add serious punch to music which in a way adds "excitement" to the music while still keeping the mids very well defined.
For vocal, classical and acoustic music, the ES5s are still my choice.
The Quads are also very forgiving of badly mastered music. A good example is John Mayer's "No Such Thing", my other headphones (ES5, Ultrasone Pro900, Beyer DT990) exhibit unbearable sibilance on that track which the Quads smoothens it out nicely. Before anyone jumps to the conclusion that it's a "deficiency" in the Quads, my workforce mixing headphones, the Sony MDR-7509HD (very well regarded among sound engineers), also smoothens out the sibilance nicely.
At this point I have to say the Quads somehow prefer to be driven direct from my iPod 5G and Macbook Pro. They sound much better direct than paired with either of my portable amps. I also have to change a statement I made in my earlier impressions - after one week of listening, the iBasso D12 ultimately sounds better than the Go-Vibe Petite. I have the Topkit on the way to me from HiFlight and I'm hoping a different combination of opamps will help the D12 and Quads synergy.
The best sound I've heard from the Quads till now is with the Schitt Asgard.
But sadly and obviously it's not portable. Also, the Asgard was not designed for IEMs so there is a hum with the Quads (not present with any of my other headphones but present with the ES5s).
I've got a pair of LCD-2s coming tomorrow which are supposed have "very good bass but recessed treble". I'm very interested to compare the two. It's not going to be a fair comparison at all but they can both be described with the exact same phrase above, so it's gonna be interesting.
From my wife's description and the frequency response chart, the triples seem to sit very nicely between the Quads and ES5s. She says it's got a balanced tonal quality, good definition and soundstage (all subjective), so I really can't wait to hear for myself. At this point I'm GUESSING the Triples MIGHT be the IEMs to consider for people who only want one pair of customs unless they have specific uses for them.
That's it after one week of listening. I've taken quite a few notes and will write a longer review soon and will include more specific musical examples to illustrate what I'm hearing.
This write-up is rather inconclusive - there is really no one clear winner - but a different IEM serves a different purpose, each having their own strengths. Use the right tool for the right job!