Hi,
I was really dissapointed of the Hiss from the AMP3 PRO2 with my IEM's, so I received a info from the HIFI Forum: use the SOUND LEVEL ATTENTUATOR form the UE TF10! I tried it, and it there really was no more hiss from the AMP3 with the TF10!!!

Ultimate Ears TripleFi 10 Pro Included Accessories - Store - Ultimate Ears Earphones Headphones Personal Monitors

Now I'm extremly happy with my AMP3 PRO2!!!

PS: Otherwise with the AKG 701 I never had hiss even without the SOUND LEVEL ATTENTUATOR!!!!

Did they fix the gain yet?

It is simply 100 ohm resistor, in my experience, it really hurts sound quality of headphones.

If the attenuator really does just add resistance in series with the headphones, which I'm sure it does, then if the impedance of your headphones is not flat the frequency response will be skewed. If your headphones' impedance is flat, then the only effect doing this will have on your setup is that your amp will effectively see a higher-impedance, lower-sensitivity load, which it will likely find easier to drive. Obviously, hiss is also less of an issue with higher-impedance, lower-sensitivity loads.

^^ what he said, and:

Even if the impedance isn't totally flat, the extra headroom will help from the amp section. In all cases using a 75 ohm resistor cable from head-direct, even several really badass earphones are tamed quite a bit from any number of amps with an extra 75 ohms. However, listening to them proves that the extra bit isn't that helpful: they lose a lot of treble presence even if the source is suddenly able to output a much better FR.

Saying all of that, my AMP3 Pro (review this week), isn't able to be rid of its hiss at all. It goes down a slight amount, but still hisses more than my other portables. The attenuator cable which comes with the AMP3 pro might be a different story and more similar to the cable which came with the Ultimate Ears - that nearly eliminates the hiss with the Stock earbuds (yes, hiss still annoys me from them).

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The AMP3 is the hissiest/nosiest source I have ever tried even beating the sound card out of my laptop. Best result I get from using ER4-S or the stock earbuds but still clearly audible noise between tracks and in quiet passages. A shame really as underneath the noise there is a very good sound.