The FD1 are very nice sounding, and in fact I am purchasing them soon to replace my Sony XBA-N3 (hybrid - DD,BA) for mostly movies as I have fit issues and love the over and in ear design of the FiiO, otherwise the N3 are technically better and priced that way.
I have come from a long line of Sony iems and really enjoy the typical healthy bassy sound, you perhaps might find the FD1 to not have as much bass as you are accustomed to. The bass is there but I don't feel it hits as hard or as deep as any of my Sonys have.
I've learnt that specs alone don't often produce the results you were expecting like with driver size, my XBA-Z5 were 16mm and I went to a 10mm driver and although there are times I can hear differences in the bass presentation the tuning of the 10mm can often hit harder. Based on specs of the 13mm driver I would have chosen the FH1s.
I'm not trying to put you off the FD1 but I don't want you to be disappointed looking for that warmer sub bass Sony sound.
Thank you for your input, I will take into account your experience!
I always thought bigger driver = more bass.