okay - i will start with some questions:
do you use the stock tubes?
if yes - the sound is not that amazing - replace them with better ones.
your fostex has an impedance of 25 ohm
- which impedance settings was your amp ordered with and did you tried both?
with my amp, my grados with 32 ohm impedance sound better with the 16-120 ohm setting than with the 8-32 ohm setting - higher impedance setting normally results in more highs, the stock tubes sound more warm to dark than analytical.
tested with the colorfly c4 the tu-05 accepts 2 volt max input voltage without any problem, a normal sign of a too high input voltage would be starting distortion.
the higher the input voltage, the better the s/n signal to noise ratio, but as the tu-05 is dead silent, you can set the player to about 66% to start with.
for the next points to explain here is a very simple schematic of a similar amp (but NOT the tu-05) with two tubes and the same principle build (single ended amp) but only for a very simple kind of comparison, experts please stop reading here):
from left to right one channel:
the first part the signal goes through normally is the potentiometer (your volume knob, here 100 k ohm). it divides the input signal voltage to a fraction of the original voltage (i.e you can adjust volume), this will result in a signal with less equal or max. equal voltage of the input signal.
here is no amplification, and as the imput impedance (here 100 kohm) is usual in the range of 47 kohm so much higher than the output impedance of the player (some ohms) no power needed, only voltage.
as there is nearly no current floating, the output power of your player is not requested or needed, only the voltage is important!
the signal then comes to the first tube, the voltage amplification stage (df91 in the case of the tu-05) where the signal voltage is amplified up to the tube anode voltage of the tube as maximum but i don't know the internal max. voltage of the tu-05).
the second tube stage (the dl92 in the case of the tu-05) normally has a voltage amplification of one or near one, decouples the first tube from the headphone but can deliver much more current, so one could say current amplification stage.
and as power is calculated 'current x volts' you now have the power to drive the headphone.
like any other highend player the rwak100 itself is designed to drive headphones (as the tu-05 is, so no big difference in output power and gain), in this case you could think of just an impedance and power convertor, but a main benefit is that the rwak battery and power is now only needed for the digital signal conversion but not power hungry amplification. the amplification is done externally by the much more stable tu-05. for example the colorfly c4 has some degradation in bass sound quality due to the weak battery at high volume level, loud music (but the quality can be dramaticaly improved by connecting the c4 to an external usb power pack).
Edited by GermanGuy - 7/19/13 at 4:40pm