I have been wondering the same thing. I also like the Micro Dac, but I am not sure which one I will end up with.
I did a search for the TC-7510 and it seems that only a few members have one, but they are extremely enthusiastic. There is very little independent info on it outside of this forum. I guess the same can be said for the 0404USB (however, it seems like a lot more people own it). The Micro Dac seems to have a lot of reviews out there, so it seems to be the safest choice....not sure if it is the best choice, though.