i just found that there are various Superpro 707 DACs - at least two different ones.
On http://diykits.com.hk/ there is one with input: USB, coax, optical and I2S, which is quite handy.
I've taken a read about the DevilSound v2 dac, the Headphonia DAC Cable and the Nuforce DAC and as i see they only accept USB as input, which is ok, however, i will probably need at least one coax input too.
Meantime the shelf has been done so now i know the exact size.
Depth should not be more than 20 cm, height and width are not limited.
Please give me some more hints.
Cheers,
David