i dont think it is hard to drive the 770pro 250.
a cmoy or meta42 will suffice.
i have a cmoy which runs on 2 9V batteries with xfeed for sale if you are looking for an amp.PM me if interested.
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MY FEEDBACK
NAIM CD 3.5+FLATCAP PS --> MPX3 --> ZU CABLE -->SENN HD 650
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