Me, with both although only about half an hour on the X5.
The DX90 does have a pretty respectable EQ but it's mainly the definition of neutrality and a wire with gain (with a slight sheen in the treble causing fatigue for me) so it won't really add much to whatever headphones you have. It also has really good extension both ends but is also ruthlessly revealing so better make sure your source files are in good condition as bassy tracks typically don't have the best mastering. The X5 from what my half an hour was worth seemed to be warmer and slightly more laid-back than the DX90 but it's still pretty respectable for a DAP. It's footprint is much bigger than the DX90's footprint though. I didn't try the EQ since I rarely use it so no impressions there, sorry.
EDIT: If you're a basshead I suggest getting an amp with a monstrous bass boost such as the FiiO E12 to pair with either of those DAPs. Apparently the E12 and X5 are a good pairing and it seems to have been designed to be so by FiiO as their footprints fit each other perfectly.
Edited by BucketInABucket - 6/5/14 at 11:27am