Just got it up and running about half an hour ago. And by that all I mean was I plugged in the power cord and plugged in the optical cable. In windows 7 it was as simple as right clicking the volume icon, pointing to playback devices and selecting SPDIF as default output. Not even a driver was needed or anything. I wish I was more of an expert but I will preface by saying I'm just a beginner so I can't give you a super detailed review. This is my first amp and the Beyer's are my first step into audiophile 'phones. Before that I had only owned some $60 sennheiser eH350s. I also only have about 50 hours on the headphones too so that won't help my case either.
After initial listen my reservations about amps quickly disappeared. I thought I might hear clipping, wouldn't hear a difference, or that it would be kind of a pointless mod like a high flow air filter on a car. Right now I can only describe the sound as crystal clear. Switching from unamped to amped the biggest difference I noticed was how much the bass cleared up. Without the amp it was pretty muddy in comparison. I think the tubedac11 has done very well at working with the Beyer's as many people have said the bass/mids are kind of poor on the DT 880 model. I probably won't be spending any more time listening without the amp but I'm sure there are several other key differences as well; this was the most obvious one in about 5 minutes. (these units receive burn in at the factory and I also bought this one second hand so I'm guessing around 50+ hours on this unit)
I'm already noticing differences in the music I know very well so I'm going to have to relisten to alot of my favorites. I also notice that alot of my 320kbps mp3 files are showing big variations in volume output? This may just be the way the track was mastered, but some are significantly louder than others. Sort of like when you're watching TV and every commercial is a different volume. I never really noticed this as much before but it seems to stick out now switching from song to song. I have the dial at 12 o clock which is somewhat loud but still very comfortable. I'm also noticing alot of flaws in recordings. Here's a song I had listened to about 15 times and never noticed it. around 0:41 you can hear a tear or skip in it.
http://www.youtube.com/watch?v=qES5dn0mmp8 Now I know what people mean by hearing all the flaws. It becomes very noticible. It will be obvious as I'm pointing it out to you but for some reason I just never noticed before.
I'm sure I'll have more comments later but these were the most noticible observations right off the bat for me having listened only less than an hour so far.
I think it helped me knowing beforehand not to expect drastic differences as someone here had pointed out the following to me;
Headphones: 50%
Amp: 15% (unless you have an odd personal tuning like massive basshead, then this may be a little more or less)
DAC: 10%
Source File Quality (below 320 vs reference): 20%
Source File Reference Bitrate(16/48 FLAC vs anything above 16/48): 3%
Cables: 2%