Tired of Racking My Brain-- Now it's your turn. Laptop vs. Roku, Slimdevices as transport

[spraggih]

It sounds different!!

Scenario A

1. Laptop via USB out directly in DAC below
   
2. Laptop via Optical out via M-Audio Transit to DAC below
3. iTunes v7.6.x, Foobar v9.5.1
   
   
4. Sound is OK but sometimes sounds harsh or grainy? Sounds fine at lower levels but begins to break up much faster than Roku, Slimdevices.
5. I *think* Foobar sounds better than iTunes.

compared to

Scenario B

1. Laptop via Ethernet out to Roku Soundbridge (similar to Slimdevices)
2. Roku Soundbridge via Optical out
3. Never sounds harsh even at high levels, bass has more impact. I can seriously hear a difference. But I cannot use the Roku due to UI and other limitations (does not play DRM music, browsing weaknesses).

Why is there a difference? Is it jitter? Electrical components?

What do you think?? Is there a laptop sound card I can get that is a fantastic transport at an inexpensive cost?

fixed equipment, software

1. Bel Canto DAC3
2. Analog RCA out
3. Rudistor RPX-33 amplifier
4. Audio-Technica W11JPN
5. Apple Lossless
6. Vista O/S