I'm not missing your point, I already stated that when sticking to the theory, you are absolutely right. But I guess I'm failing to get my point across.
My point is that there is more to it than just soundquality. I'm not talking about electronic engineering, I'm talking about how the listener experiences things. It's all about finding a balance between all factors to create a sound that suits you. That's why I said it made a difference to MY ears and was talking about MY overall listening experience. In the end it's all a matter of perception.
I know this isn't anything scientific, but if how a listener experiences his music, is expressed in points, a base number (eg 100) and changes to the setup can either add or subtract points (and more points is a better user experience).
Let's say the the Clip+ & Momentum setup has 100 points.
Now I introduce the E6 in the mix, yes, the soundquality will be less, but not a whole lot, you wouldn't hear it unless paying attention to it and most people with 'untrained' ears probably wouldn't be able to tell the difference at all, so let's say this is -5 points to me. But the added power (a difference of 15mW vs. 150mW at 16 ohm) of the E6 to drive the Momentum has an impact on the sound which I like so much, that I give this change +10 points.
You do sacrifice some soundquality, but overall the listening experience gained 5 points. This is ofcourse all a very personal matter. It could very well be the other way around for someone else.
But isn't this is why we share our experiences on forums like head-fi? To learn from eachothers experiences? I have found a few headfiers who's tastes match mine pretty closely. If they share their experience with a certain setup, I pretty much know immediately if I can buy it blindly, need to try before I buy or not bother with it at all. This has saved me from making a few purchases that I would have regretted. And as I have a very limited budget for this hobby, that's a big deal to me. So I share my experiences, hoping that it might help out someone else too.