Originally Posted by stv014
It probably still has the same problems as the STX, like the output impedance of the headphone amplifier, and noise with very sensitive headphones/IEMs (due to the lack of analog volume and gain control), maybe even the increased noise floor at 44.1/88.2/176.4 kHz sample rate is still there.
This statement got me curious and at first I thought you maybe experienced a defective unit, because I use an HD 650 and even at 600 ohm setting getting the volume slider on max produces zero practical noise. But to check your statement I plugged in my Xperia PLAY in-ear headphones and at 300 ohm on maximum volume a background noise is evident, low volume, but still very noticeable if no music is playing.
But then you have to remember, this is not a sound card designed to drive low sensitivity headphones. At 600 ohm, this is a monster sound card to drive brutal diaphragms. If you want to use low sensitivity headphones it's probably better to stay away from a monster card like the STX.
Not everyone will agree with what I'm saying, thinking that a good card will handle any headphone with perfection, but there is a fact in the engineering world that is inevitable: if you gain too much on one side you'll also lose on the opposite. The fact that the STX performs with incredible performance on higher end headphones and has background noise on lower end headphones shows that the STX was clearly engineered towards high end, and I accept that. I won't use my cellphone in-ear phones on my PC.
The noise is not a problem on this card if you use a headphone recommended for a card like this. I call this "using a product the way it was intended to". It's the same as trying to use 10" wheels (headphones) with an off-road vehicle (sound card). It will work, but with clear limitations.