So after listening to a few sets of headphones (lcd-2, pfe 012 iem, ehp-100 iem, ue6000) through the speaker taps of my arcam, I would say there is some improvement in the sound. Dac used was cambridge audio dacmagic. Most notable to me is that the highs are less fatiguing (something I'm very sensitive to due to a problem with my ears). I cannot listen to music at any sort of volume these days, but I was able to enjoy a perceived louder volume without pain in this setup.
The above I can guarantee as the presence or absence of pain isn't easily confused, the following impressions may or may not be placebo:
I felt the music may have had more energy and been generally more engaging. This was actually particularly true with the phonaks, which in this setup had mids and highs that blew the yamaha iems away (lively and crisp), something that was not so significant when I compared these iems out of a cowon pmp or the o2 amp. The lcd-2s sounded to me a little more open, and responded well to eqing the bass (which they certainly don't from the o2 amp). Normally to listen to the lcd-2 for any length of time I have to significantly eq down the highs, particularly 12khz. I enjoyed the lcd-2 uneq'd alot in this setup, it was immediately obvious that I was listening to music in the highest quality that I had yet heard.
Yes I had some pretty sensitive headphones connected to the taps of this 100WPC amp, the arcam has volume controller with fine steps which made this possible. I didnt't use resistors. There was a noise issue with all these headphones, though with the lcd-2 it was barely audible without music and completely inaudible once music was playing.
What I took away from all this is more of an idea of how much an amp can affect the sound. Highs became so much more listenable, and I definitely wouldn't say it was because they were less present. I have a feeling the DAC makes a bigger difference though, as when I remove the dacmagic out the chain and use onboard laptop DAC, the sound degrades considerably more so than the amp switch improved it. The dacmagic isn't a great DAC either.