My audiologist was a hearing aid specialist too... indeed, I was his first ever customer that requested impressions for CIEMs.
Anyway, I just came across some impressions I sent to Flysweep some time back (before he pulled the trigger), in response to treble. Potential buyers may find it of interest:
"I'm still very much enjoying the AUD-5X. With regards to treble, my context is limited due to the relatively short amount of time I've had this interest, but I'm sure you'll take that into account.
I'd say that of the phones I've owned, treble quantity (relative to the bass and mids) is closest to the GR07, albeit a bit more forward. Not at all forward like the DBA-02 or FX-700, but also worlds away from the recessed SM3 or ASG-1. The balance achieved in quantity is to my ears, near on perfect.
With regards to quality, I'd say they're technically on par with the DBA-02s and FX-700s, albeit smoother and less fatiguing, whilst losing no detailing. Texturing and layering with the Dreamearz exceed the other two by quite a bit, which is somehow accompanied by greater coherency. Whilst I'd say that the bass, mids and soundstage set a new standard to me, the treble did not. It is however still easily up there with the best I've heard.
I've tried 2 aftermarket cables so far, and am about to receive a third. The Silver Dragon added a touch more sparkle, with noticeable improvements in air, space between instruments and sounstage depth / width. The other was DIY SPC toxic cable (see thread for pics) which sacrificed a bit soundstage depth / width, as well as sparkle, for a greater sense of warmth, impact, and elevated mid-bass. That's not usually my sorta sig, but I really liked it with the AUD-5X, preferring it to the SD. My next is going to be a pure silver toxic cable, which should prove interesting!"