In my experience, tubes can add depth, amazing layering, bloom, body and some add warmth (all of which good SS can also do), but I also think tubes with a digital source make the sound somehow artificial, synthetic, if you will, and am going back to all SS gear on an all digital front end.
On a side note, I recently heard an all McIntosh setup, with tube amp and preamp, McIntoshch cdp and speakers, that sounded so much like solid mid-fi solid state reproduction I had to make sure the speakers where in fact hooked up to the claimed components. It's just about tradeoffs, you pick your compromises. I have found SS gear to sound more real, more believable, less synthetic, but can see where the appeal of tubes come from with a digital setup, especially with a hot tweeter and limited range speaker, where only certain types of music is listened to.
On a side note, I recently heard an all McIntosh setup, with tube amp and preamp, McIntoshch cdp and speakers, that sounded so much like solid mid-fi solid state reproduction I had to make sure the speakers where in fact hooked up to the claimed components. It's just about tradeoffs, you pick your compromises. I have found SS gear to sound more real, more believable, less synthetic, but can see where the appeal of tubes come from with a digital setup, especially with a hot tweeter and limited range speaker, where only certain types of music is listened to.