So, I need to retract some of my initial findings and impressions!
First and foremost, the RS2e's sound definitely better out of the Duet. The first day I got them, I started out with the Duet and spent a good while listening out of it, then switched to the iPhone 5S. This did, I felt at the time, tighten up the bass a bit. However, over the last few days, I have definitely not been enjoying the RS2e / iPhone 5S pairing! It becomes fatiguing after a while, more or less, depending on the music and the state of your ears and mind. The RS2e's have really shown me that the audio codec of the 5S is pretty harsh and "digital" in the treble. The Sennheiser HD-595's pair well with it precisely because the treble isn't forward and is even recessed in a few places. I'm more susceptible to getting this listening fatigue at some times than others, and recently is definitely one of those times!
Today I was trying to track down this issue, and was again listening to all my headphones with a set of test tracks, and finally decided that the problem wasn't that the RS2e was emphasizing treble regions, but that either the DAC or the amp or some combination of both in the iPhone 5S was harsh and the RS2e's were just revealing it. So, to test that I switched back to the Duet which I hadn't listened to since that first day, and immediately the harshness was a non-issue, and everything was so much better--except the bass, which was
maybe not quite as tight. I don't know why I felt the iPhone was "definitely better" that first day. Probably I was caught up in the excitement and thrill and joy of these headphones at the point that I switched over, and my ears were "warmed up" so to speak, so that when I switched over, I was just noticing the slightly better bass and didn't notice the more fatiguing nature of the sound.
But I have certainly noticed it lately!
Clearly, my downward spiral has started in earnest.
I am now definitely going to get a dedicated desktop / full-sized headphone amp so I can use my Duet as just a DAC and send it's line level speaker outs to the amp.
On the upside, I can also now say that the comments I made about Adele not sounding as good as other artists did on the RS2e's was an artifact and consequence of the 2e / 5S pairing. Out of the Duet, Adele sounds freaking amazing on these RS2e's.
So I've decided to get one of the Schiit amps. I love their topologies, the all discrete components, the made-in-USA, the look, the reviews, and their refreshing irreverence. I have an Asgard 2 on order (it's backordered at the moment), but I'm wondering if I shouldn't switch to the Lyr 2.
A couple of the reviews I've read had positive things to say about the Asgard / Grado combo, with one reviewer (6 moons, IIRC) going so far to say that he would package the Asgard with Grados! LOL! Plus, from everything I've read, I think it's quite likely that I'm a solid state guy, so starting with the Asgard 2 seems to make more sense than jumping right to the Lyr 2, which would kind of be overkill for the Grados, power-wise, not to mention being $200 more expensive. But perhaps that's where I'll end up. We shall see.
However,
if anyone feels strongly that the Lyr 2 is the much better pairing, please chime in! I'd love to hear what you have to say. Thanks everyone.