@Anaxilus: Kudos for the measurements. I see that you strapped the hardest IEM load ever to exist*  to the amps and tested with a dynamic driver IEM only once. Care to measure with a more normal load too? Also, what are you measuring into?

Seems that RMAA is doing something weird calculating noise levels too. E.g. Leckerton UHA-6 is about -110 dB(A) given the picture, but it says it's -90 dB(A). I wouldn't trust it for anything after that. Pity actual signal analyzers are expensive. However, a good software analyzer should work well. (Either dScope or TrueRTA, neither is free obviously.)

*Triple.fi 10, which dips to 6 Ohm at highest end, making anything with nonzero output impedance roll off. Also high capacitance, so many amps have degraded IMD at highest end. Also, there might be something even harder to drive around.

I'd assume that's the point of using something like the TripleFi.  It's supposed to be hard.  Also, since RMAA doesn't calculate Zout you use a load like the TripleFi to infer it.

Not many portable amps have crossfeed on them.

I've heard the Corda2Move's crossfeed. Hardly impressive.

The only other crossfeed amp I've heard is the Phonitor. Now the Phonitor is something else....   but at 2k..  lol..

Well if I'm stationary I don't really need it as much since I can use TB Isone with anything I play through foobar and I use a multichannel to binaural DSP in ffdshow for movies.  With other stuff on my PC that I can't route though an arbitrary DSP like web streaming or games I've been using my UHA-4 instead of my ODAC to add the crossfeed.

I just got a bunch of film cap in today though so I'm going to make my own crossfeed filter.

Maybe means nothing, maybe means everything.

http://scientistsaudio.blogspot.jp/

What's the Zout on the MKII?  I don't see it on Nick's site.

I've asked about it via email, it's < 0.5 Ohm.
One hand-built O2 I've tested had 1.25 Ohm.

That 0.75 dB difference on O2 w/ TF10 might be just about audible...

Hmm...

The O2 is supposed to be .5 ohm too but with hand building I guess there's more room for error...

Actually the 0.5Ohm Zout on the O2 can cause problems with BA's. That's actually one of my major gripes about the amp (that and the sound sig). This contradicts the promise that it will drive perfectly any reasonable dynamic headphone.

P.S. The zero ohm Zout is an ideal concept that doesn't work in reality that way. You can get extremely low Zout but ultimately you get limited by cable/contact surface impedance.

You'd need a pretty wild impedance curve, even by BA IEM standards, for that to be audible.  Is there anything like that?

By problems I mean low damping factor. Flabalanche.

As for impedance dips, most likely they won't be audible.

Do BAs even need good damping factor?  It doesn't seem like it would be that important to that kind of design.  It's never something I've noticed either (at least over the FR aberrations...) and adding series resistors to manipulate the FR is somewhat common among both manufacturers and users.

I mean the ER4S is just an ER4P an extra 75R or something in series to bring up the treble.  Is the S a 'flabalanche' compared to the P?  I'm seriously asking, I've never heard either.