I hope you wind up finding the F as enjoyable as the X... It'll run pretty much any earphone you want to use with it. But I think it rewards for using high end ones, because the sound is pretty refined. I don't find the bass that much less than with the X, but that is with the Fitear TG!334 earphones which are highly revealing of everything in the frequency spectrum. You'll find a more expansive/open, and detailed sound over the X which sounds a bit more thick and congested by comparison. I still have my X, and have done back-to-back comparisons many times, and always hear it the same way. The bass is just a touch less thick, but tighter, and reaches pretty close to the same depth. I think what you may be hearing is that the X presents a slighter thicker bass (maybe it's more mid-bass in the mix)... that's how it seems to me anyway.
But you should be good with just about any high end earphone you choose. Westone 4, IE80, Fitear, etc etc... will all be good.
Enjoy.