I don't think I need a USB DAC. I would be using the amp to power MTPCu headphones. The source would be an Ipod Nano using a Fiio L9 line out adapter. The E7 was recommended because of its low noise, which the MTPCu are particularly sensitive to. So maybe the E11 is better, as you say, and really I should be asking to compare the E11 to the T3?
For sheer bang for the bucks, I will recommend E11 over T3D as I find they to be running very close to each other (see my review of E11 here). Some said T3 is a little better than T3D, though I didn't notice a lot of difference when I have the chance to briefly audition both side by side. I didn't notice any hiss on MTPC with both E11 and T3D, just so you know.