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Am also very satisfied with the pairing. (However, am a newbie in "head'fi" things and iems in particular. I usually listen at home with b&w 804 nautilus speakers and a marantz sr7007 amplifier.) I actually bought the ibasso dx50 after realizing my cheap mp3 player sounded crap even with the re-400. It sounds infinitely better now. (I also did about 50 hours of burning-in of the re-400.)
(By the way, thanks for all the forum entries on headfi, I have been reading up here and on other websites for 2 weeks, hesitating whether to get the fiio x3 or the ibasso dx50.)
One question: The one thing I couldn't find despite crawling through the forums here (including the bugs page for the dx50), is a clear explanation of how to make the dx50 (fw 1.2.2 manually installed) play tracks in multi-disc albums in the proper order. Right now e.g. my Satie piano works (5 cds) are especially messed up... (Am using foobar to rip my cds to FLAC files and to do the tags...) Should I re-tag them on foobar as separate, single-cd albums?
I haven't played around with many multi cd albums yet, but the ones on my sd card play correctly. I used Mp3tag for tagging after using foobar to rip the albums to FLAC and simply denoted them as disc number/total discs (such as 1/2, 2/2) in the discnumber field (as the format defaults to). My song titles go by the "track number - title" format with leading zeros on the single digit tracks and the songs show up back to back in the order of the discs under the single album.