Are looking for USB only as in no AC power either?
I think it is hard to find a meaningful upgrade going from an entry level DAC to upper-entry level DAC especially sometning usb/battery powered. I have 200.00 worth of USB powered DAC/AMP (D4) and do not plan an upgrade for my desktop for a while (getting new portable amp soon though).
With that said.. I like my Ibasso D4 DAC/AMP very much Not sure if it is a big sound improvement over the e7 (although D4 reviewed highly here) but, it has rollable opamps so there is a major functionality improvement (you can tailor sound).
I listen to metal too (older metal, Sabbath, Priest, Maiden) and I felt the topkit rollable amp kit from HiFlight improved the sound of the heavy, faster paced songs (such as after forever).
The D4 is both a very good DAC and a very good AMP. I do not see myself upgrading until I go with an AC powered DAC (not usb powered) in the high-mid to high end arena (800+).
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Edited by Rat Salad - 7/7/11 at 5:59pm