OK - so I'm pretty confused now.
I took mikeaj's advice, and conducted a little test tonight. I used my Q701's and my DT880 600 ohm. I don't have an SPL meter - so I had to carefully volume match as close as I could by ear. I used iTunes with no EQ (I'd usually use Foobar - but I wanted to be as close as possible to the iPhone, so figured 'why not'). So was the test conducted using proper volume matching - no. Was it sighted - yes. Hardly convincing or scientific so far huh :)
Rather than use FLAC - I used aac256 for both set-ups as personally I can't tell the difference from lossless anyway. I'll also add that I have low level permanent tinnitus - and definitely no golden ears. I'm well aware of my personal deficiencies - and have no problem with the fact that my hearing is likely not as good as many who frequent these fora.
Anyway - I tried to use tracks that I know fairly intimately, and also some that highlight different parts of the sonic spectrum.
- PC (iTunes) > optical > NFB-12 > high gain and about 10-11 o'clock on the pot > DT880 600 ohm
iPhone > DT880 600 ohm (volume at roughly 80%+)
- PC (iTunes) > optical > NFB-12 > high gain and about 9 o'clock on the pot > Q701
iPhone > Q701 (volume at roughly 60-70%)
Again - all of this was done sighted, and I don't have a switching box - so it might have taken me about 5-10 seconds to switch between devices.
After a lot of testing 40-45 minutes my observations:
- Bass on both headphones wasn't an issue - on either set-up. It sounded pretty good actually - even on the 600 ohm Beyers via the iPhone. Via the NFB-12, bass hit and definition was (to me) slightly better defined - but we aren't talking major (night and day) differences. Surprised me actually.
- On most of my modern music there didn't appear to be any issues with peaks (no clipping I was aware of)
- Only issue I had was when I switched to classical and I started running out of headroom with some Tchaikovsky. Had to turn the volume up with all set-ups, and with both iPhone set-ups I got audible distortion - guess this is clipping right? On the NFB12 - I was at 1 oclock with the Beyers and about 11 oclock with the AKGs (both sounded fantastic on the desktop set-up).
- AKGs in upper mids and treble lacked a certain 'something' in some tracks when unamped. Hard to describe. Not as clear / lively. Wouldn't discount this being placebo though. Again differences not large.
So now I have to eat my words from the previous pages - I'd love to go back and edit what I originally wrote, but I spoke up without volume matching previously - so I need to publicly admit how wrong I was.
I think in a blind test properly volume matched - I'd be lucky to tell the portable from the desktop set-up on most tracks. There is no huge deficiency in the music - and the only real issue is with music that requires more volume - that the iPhone struggles to reach.
I must say - I now have an even bigger respect for both my iPod Touch and iPhone as a DAP. I guess I'll still need either a small amp for Mum to go with the K701 and her classical - or a better player. If she was into classic rock that wasn't too dynamic - I honestly wouldn't bother with an amp after what I've learnt tonight.
Anyway - thanks all for the help.