For me, IMO - sound quality that is measured from 1 to 100 percent of perfect - the headphones affect the quality in multiples of 10 percent while the DAP affects in multiples of 1 percent. However, music file quality can also have quality effects in the 10's of percent range depending on file resolution, recording, mastering, song/performance quality. Going from mp3 to AIFF will likely have the effect of having some MP3 songs that sound really good but having most sound average or bad on your playlist to AIFF having some songs that sound bad but most step up to sounding better than average or great. The MP3 ones that sound good may sound even better in AIFF format if you have a player that can support HIRES properly. Only then does the extra multiples of 1 percent that a DAP adds can even be heard. I can even hear the difference in file format on my lower sound quality iPhone 5 and 5S. Converting to a AIFF file format was the greatest gift I gave myself before even buying good gear and was what got me started in looking for what was possible in improving sound quality. Remember that this sound quality range that I am talking about is not additive, but limited by the lowest sound quality item in your chain. So if your music sound quality is figuratively holding you down to say 85 percent of perfect, then your Roxanne paired with the highest quality DAP will still be limited to 85 percent.
You already have a TOTL CIEM so you have a great basis to hear high quality sound in the 90 percent range unless you are being held back by the music quality. Do yourself a favor and try burning some of your favorite CDs to AIFF in iTunes and give it a try. Remember, it will not make poor recordings sound great, but will make more of your collection sound noticeably a little better - over time as you get used to it - taking your music quality it to the next level. You will start to hear new things in your music and start to get drawn in a little deeper and easier to the good recordings. Again, remember that crappy music that is mastered and recorded perfectly and placed in AIFF format will still sound like crap so be choosy.
As for the DAP, if the rest of your chain is ready for high quality sound, I don't have the Roxanne, but my Hidition NT6pro CIEM is very noticeably better paired with my X5 and my DX90. It takes them to the next level providing more drive, clarity, and texture that you can feel. However, with a DAP it is all about pairing which you will only know when you audition them correctly. My X5 and DX90 are easy picks providing best bang for the buck as exclaimed on Head-Fi so they are a safe starting point. FYI, my X5 pairs a little better with my NT6pro than the DX90 as it has a little more power and blacker background that pairs well with the NT6pro signature. As a point of interest, I'm not much of a believer in DAP burn in, but the DX90 seems to be catching up in quality over time where it matters less which I use.
Good luck in your choices.