First a little intro of myself: I am a pretty new head-fier. My initial interest in head-fi arised from the discovery of how the higher end earbuds (e888, ex70) were superior to my stock pcdp earbud. Then I discovered the eye-opening head-fi forum. Now I am fully convinced that investing in a moderate source, a dedicated head-amp and a fairly a high-end headphone is the cheapest way for more than decent sound. :-)
Since I value portability, I has been eyeing on the airhead/total airhead/porta corda II to be my first head-amp for quite sometime. I was reluctant to buy online and I was happy to know I could try out at rameish's place before purchasing.
I find the porta corda suits my current portable/home need with marked performance improvement (against without a delicated head-amp :-) ). The improvement is noticable in the tighter bass and better resolution with my HD590. There is also a switchable crossfeed function that seems to reduce my fatigue for extended listening. The effect varies across my CDs and headphones but it does make the sound generally more natural. (A side track: Don't you think that cans like my HD590 have some crossfeed by design that they are so open? :-p )
The porta corda is also quite a configurable device and let the end-user choose its battery consumption by choosing between class A or class AB operation and choosing whether the "on" LED should be connected. Personally I prefer my device to perform the best it could and have the porta corda switched to class A operation though according to the menual it halves batter life.
Another more troublesome "configuration" for the porta corda is to attach it to AC power supply and change the supply voltage. This involves buying a regulated AC-DC adapter, soldering the fitting plug to the power-supply and drilling a hole at the porta corda. It was less troublesome for me because Rameish is happy to do the drilling and soldering for his customer. (Thanks, Rameish. :-) )
The configurations cannot make quantum jumps in sound improvement. Personally I need to be attentive to hear the improvement with class A operation and 15volt of input voltage. For home use I won't mind saving battery power and pumping in extra voltage into my porta corda from the AC socket.
One interesting accessory that come with the porta corda is a 120ohm adaptor. I guess this is a similar device as the Etymotic 4P to 4S adaptor (that is 80ohm though). I spent quite some time trying it out and comparing the effect with and without the adaptor. I think it generally produces warmer sound but with slight reduction in resoltion. I prefer it with both my HD590 and Etymotic 4P.
A note on the service of Rameish: I would say it is great. In total I think I have taken 8 hours of Rameish time for selling me the Porta Corda: 5 hours auditioning the porta corda before the purchase and 3 hours to do the soldering, drilling and let me do somemore listening afterwards. A nice guy to talk to and learn from for a head-fi newbie like me too.
I find that the porta corda is a keeper that even I when I upgrade to a real home headphone system in the (unforseeable :-) ) future I shall still retent my porta corda for portable use. Meanwhile, I have had enough of equipment purchase. Let me enjoy the music.