USB-S/PDIF Converter or USB DAC?

I am intrigued by USB as soooo many posts claim its superiority, but can somebody please explain this:
-Isoncronous (isosyncronus?) data transfer..
-and bidirectional with error corrrection.
-We also have the new DVD players with buffer systems to do the error correction,
-then we get the external clocks like the BigBen or the DCS verona that enslave transport and DAC to the same clock, so no error from 2 clocks there, supposed to be great also!
Everyone claims to be the best...
What about a huge overbuilt transport like VRDS, CEC or Forsell...

I would only trust my ears because there are a lot of technical explanations that sound soooo logical but when the time comes to hook the thing up to your system they just dont sound that good...
I had in my system Squeezeboxes modded and not which have Ethernet connection, I dont think you can go more bidirectional than that! Unless they are not using the function (isoncronous maybe?) It does sound nice and very comfortable BUT not audiophile quality really (compared to say a Forsell transport)
I had a intensly modded DVD player with the buffer thingy and extra clock etc, it just dont cut it (compared to say a CEC transport)
I have yet to try the external clock and a USB DAC but I am getting less and less motivated by this findings...
I am really intrigued by the Wavelenght DACs mostly. I hear a lot of good things about them!
Anyone compared some of these in their own system?
Thanks

Thank you very much for your answers Steve and Hagtech!

Does the lower jitter correlate to better sound...I am dubious there...

How does the Squeezebox works? Is it bidirectonal? I know it has about 5 minutes of buffer, does that help with jitter? What are the jitter figures of the Squeezebox2?

I have compared the Squeezebox2 to a transport and it was not even close, yet the jitter is lower in the SB2... Again: Does lower jitter means better sound?

All the Best