I've had the XBA-300's for a few months now, and even though I have gotten used to the sound, it's just no comparison to when I switch back to the XBA-30's. The XBA-30's are unmatched in terms of having an even signature with no major spikes or faults anywhere along the entire spectrum. Yes, the extended highs are slightly boosted, but there are barely any IEM's in existence that can do highs as smoothly and as detailed as the XBA-30 below $300.
In my opinion, what the XBA-300's do "well" is sound somewhat closer to dynamic headphones. They've toned down the 10khz+ sparkle that was prominent on the XBA-30's, and they have a harshness probably somewhere above 6khz that, unfortunately, I think some are too willing to attribute positive qualities to. Basically, the XBA-300's sacrifice rich mids and lows for sibilant highs. I'm sure they are great for some types of music, but they are not like the XBA-30's, which are some of best all-rounders ever made. In fact, I think the XBA-30's were so good and even that maybe some people didn't recognize them as such, because were too unusually good. It could be that their ear canals are different from mine.
I've owned a couple of pairs of IEM's, from $50 to $350, and my two favorites are still the XBA-30 and XBA-40 after many years. I have given other headphones a chance, and I am dying to find the next true upgrade for when the inevitable day comes that they break. Here are some of the ones I've heard:
MDR-EX600: Somewhat natural-sounding, but overall pretty bad.
XBA-H3: Unpleasant humped bass (the type that is praised for being "fast", which means they are overemphasized around 100hz-200hz and roll off a little bit in the sub bass), sibilance, veiled mids. Returned.
XBA-A2: Very nice and smooth mid to highs, overpowered by even worse unpleasant humped bass -- bass so 1 note and punchy, I couldn't get used to them even after listening for hours. Returned.
XBA-300: Harsh highs.
Klipsch X10: Very rolled off highs, since they are 1BA. Gave away.
Monster DNA IEM: Bought on sale, super flabby bass. Gave away.
Philips Fidelio S2: Somewhat decent, but sibilant and unforgiving, and not high resolution since they are dynamic drivers. Returned.
Yamaha EPH-100: Super piercing highs. Couldn't get a good fit, and one sided shorted out on the first day. Returned.
Here is a frequency response graph for the XBA-3's, which people say had less refined highs than the XBA-30's (Goldenears never did a graph for them):
I did not find one pair of IEMs on GoldenEars that had a more reference-like graph than these. Even though a graph doesn't tell you everything, I think my ears mostly agree with their neutral target. I am certain that if they do a graph for the XBA-300's, it will reveal the harshness I am talking about.