My HD8 DJs showed up on Saturday along with the Vali amp. Without the amp, they were good, but not worth $400. The bass was pretty good, but it wouldn't ever go loud enough, because of their high impedance. They were comparable to my HD 439's, but with more defined bass. When I hooked it up to the amp, though, it performed significantly better, and the $400 price tag started looking a lot more reasonable. The bass was very prominent and clean. Songs with kicking bass really kicked.
I brought them in to work, and everyone was impressed. My setup outperformed everyone's gear by a long shot (obviously, especially since most people had Skull Candy). I had people from all over the office coming over to my desk to try them out. Most people weren't comfortable with turning the volume up to rumbling bass levels, though, but they all loved them. Seriously, though, if you turn them up loud enough on certain songs, it rumbles. My whole head rumbles (yet it's still clean without any distortion)! It's insane. A bunch of people brought over their headphones and tried them out on the Vali. All of them sounded much better with the amp, even the low impedance ones (and they were all over-ear headphones).
I'm big into bass, but I like it clean, and I never want my bass to distract from the highs, which the amp helped facilitate (without the amp, the highs were quite lacking). While I was at work today, my Modi DAC was delivered to my house. I just got home, and I got it all set up. My first thoughts were that I love the way the Modi and Vali stack and look - very professional. As for the sound, the Modi helped bring out the highs a little more, and everything just sounds better. It's not a huge difference, but it's definitely noticeable. I tried the Modi + Vali combo on a lot of my headphones I have lying around, and I can say that together, they make anything sound significantly better, but I'd say that they especially made a difference with the HD8 DJs.
Overall, I'm quite impressed and pleased with the HD8 DJ + Modi + Vali setup. I would say that the DAC isn't strictly necessary unless your source is noisy, but that the amp is a must-have. I wish I could try the HD8 DJs on a Magni amp, though, because I'm sure solid-state would produce quite different results than the tubes in the Vali. One interesting thing to note about Vali and other tube amps is that they're sensitive. If you tap on the top of the case, or if you bump your desk too hard, you'll notice that the amp produces a tone that lasts for a few seconds (so don't plan on using the Vali in your car or anything, but if you listen to your headphones at a desk all day, you'll be fine). Also, there's a very faint hiss when using the amp. It doesn't affect the music at all (because it doesn't change with the music volume or even with the volume knob). It's a constant level, and I can barely hear it anyways (and only when no music is playing).
Based on my limited experience, I'd recommend the setup I have, but again, I haven't tried out the Magni amp. I can tell you that the Vali does go much much much louder than you could ever want with the HD8 DJs. Running through the DAC, for my normal listening volume, I have it at only about 30° above the lowest setting. At around 90°, it's about as loud as I can handle without feeling myself go deaf (mind you everything still sounds just as clear at this level), and the nob goes all the way up to 270°.
So take what you will from this information. I can only tell you that the setup I have is great and works awesome. Perhaps there's a better combo out there, but I'd say that you can't go wrong with what I'm using, and it's all a relatively fair price.
Let me know if there's anything else that you'd like to know. Oh, and if you end up going with the Magni, let me know how it sounds.
Edited by andrewLarsson - 4/7/14 at 9:25pm