I just posted the following to my review of the iCAN amp here. It might be of interest to anyone using the HD650s that wants a bit more detail to the sound:
Last night I plugged my DT770s into the iCAN now that they are more broken in, and I was pleasantly surprised at how clean and detailed they sound, without veering into the edginess that I heard with the Beyer T1s. Pretty good for a headphone that costs over $1000 less! They pair up well with the iCAN.
But then when I went back to my HD650s there was still that nice warm tone, but now they seemed to lack that last little bit of detail, and I am kind of a detail junkie. I just love hearing every little texture and ambient sound. So I pulled my TDS202 (http://www.amazon.com/Acoustic-Research-TDS202-Enhancement-1-Speaker/dp/B00005QZ16) out of my living room system and added it to the chain. Boy does it bring out the details! It was too much with the DT770s, but with the thicker sound of the HD650s, it added a whole new level of sharpness and clarity, even above what the 3D effect does for them.
So if you have a set of headphones that you like, but that always feel like they lack some of that low-level detail retrieval, and some treble sparkle, then this little gadget can do wonders. It can do this with any amp, not just the iCAN. It also adds even more spaciousness to the soundstage when combined with the 3D effect on the iCAN,so should do the same on any amp.
It is probably not audiophile approved, but it is still fun, and it is only $30. I will have to see how it wears on me, as in other headphone systems, I found it got tiring after a while. But I think the sound coming out of the Tera Player is so much cleaner and smoother than anything I have ever used as a source in the past, that I might find I can use the TDS device without it wearing on me. It still sounds wonderfully clear but still warm and musical this morning. However, iIf there is any noise in the system, like hiss, that also gets boosted. Here is a review of an earlier discontinued device that used the same technology:http://www.stereotimes.com/acc110198.shtml
And here is the website of the developer of the technology: http://tdsaudio.net/testimonials/consumer.htm
Note: There is an error in the title on Amazon. There is no speaker in the unit, and you only need one for hooking up two channels via RCA inputs and outputs. It also comes with two pairs of AR RCA interconnects in the box.