So far I am absolutely enjoying it. The DAC and built-in amp is very clean, it is neutral sounding and detailed.
It did not make my 900s any brighter, bass is more controlled and has more impact compared to the Fiio. Especially with faster bass n' drum tracks.
You notice this quite easily when there are a lot of instruments being played at the same time, combined with low frequency bass. Less soup and hi beef.
I was originally afraid that the 900 would be too bright by not getting a darker, warmer sounding unit.
Seems like I took the correct decision.
The 650 has the same results. The Quattro does not add any coloration, it lets you experience the true sound signature for your phones, in the quality they are intended.
I don't have more than 30~ hours burn in, but I am enjoying every minute.
Looking forward getting the amp and run the Sennheiser's balanced ;)
Oh, my Setup is PC > SB Touch > Quattro.
Haven't tried the USB yet for music, except gaming.
PS. I have had limited exposure to High-End equipment, so I can't really compare it to anything else but budget.
This is the best dac, amp product I have ever owned, and the most expensive.
Edited by zoqi - 8/5/11 at 11:43am