In case you've not seen this Headfonia review of the Fiio E10 I would check it out... http://www.headfonia.com/the-latest-must-have-the-fiio-e10-usb-dacamp/   They seem to think the E10 is substantially better than the E7.  Versus the E7 they say this about the E10:   "Comparing to the...

is it really necessary to get both the E7 and E9 for the HD598s?  Wouldn't just one be sufficient (ideally the E7 as its a bit cheaper..!)  Given that the HD598 is $180 it would seem a bit overkill to spend more than that again for E7 & E9?   Tips from those who have both and whether the...