Bear in mind, you will be double amping from the U3. All you're doing is amping the amped signal with a better output impedance, which will stabilize the X1's frequency response.
Honestly, I'd just get the U3 and x1 for now (though it may sound a little thin at first), then buying a used E17 later. We're talking like a $20 difference from something like the E11 anyways.
The E10 only works as a usb dac FIRST, so you can't connect the U3 to it.
The E7K IMHO just isn't worth it, when the E17 is just a little more and has all the features plus more power.
Edited by Mad Lust Envy - 1/6/14 at 3:08am