USB vs s/pdif

I'm using a Wyred 4 Sound DAC 1. It has the usual compliment of inputs, including USB. I connected my MacBook via the USB input and was surprised to hear how bad it sounded. At least compared to the s/pdif output of my Squeezebox 3 with Bolder digital mods. There was really no comparison. The USB/MacBook sounded flat, lifeless, constrained and not very extended at the frequency extremes.

Why is that? Is the SB3 that good or am I doing something wrong with the MacBook?
I'm pretty sure that the USB input on the Wyred DAC1 is adaptive, not asynchronous. Implementations of adaptive USB vary widely in terms of SQ, largely as a consequence of jitter. I haven't heard the USB implementation on the DAC1, but it wouldn't surprise me if it's less than stellar.

The Wyred DAC2, however, has an asynchronous USB input. I have compared its USB input with its S/PDIF input and I found them very, very close in SQ. (I don't own the DAC2, but I borrowed it from a friend to listen to in my system).

FWIW, I believe that Wyred will upgrade the DAC1's USB input to asynchronous (like the DAC2) for a charge.

I keep trying USB interfaces on DACs and I just haven't heard one I think is very good. I have heard the USB interface on a Wyred4Sound DAC2 and compared it to spdif and toslink. If I owned that DAC I wouldn't use the USB interface, just like I don't use the one on my own DAC.
I'm using USB with a moon 100d which is asynchronous. Moving to a decent USB cable made a lot of difference. Tried toslink, which was OK, but I prefer the USB route.
My experience with USB was always a letdown until the async USB of the W4S DAC2 I'm currently enjoying.
In my opinion, unless asynchronous, USB is not up to par with S/PDIF. In asynchronous version, the cable stops making a difference, as no timing info is passed from the transport to the DAC.
I'm sure other posters will have different opinions, though, as usual.