HD650s are not meant to be driven with a DAP. Will you get sound? Yes. Will you get loud sound? Yes. Will you get good sound? No. Unfortunately people are mislead in this area. A set of phones like the HD650 (which I own by he way) require a dedicated quality "home" setup to really reach their full potential. It bothers me every time I see people doing exactly ehat you are doing because somehow, somewhere you were mislead to believe it is all good. Even my most powerful portable amp the RSA SR-71A (which runs on (2) 9 volts batteries!) does not make the HD650's shine.
Thanks for the comments although I'm not really mislead just did not give a full extent of my understanding and situation, I am far from in the dark. I know that the general consensus is that a dedicated amp is needed to best drive high impedance cans from being a head-fi.org lurker.
I will use the sflo2 with mainly M2 IEMs and HFI-780 ultrasones which are only 32ohm which should deliver full potential. The HD650s will primarily be for my home seup PC (Asus xonar DX) + new amp yet to be decided (Pico slim / ibasso D4 / Fiio E7 / mini dot / headamp or cheaper alternate ) integrating the Sflo2 for music.
Although Im sure that there are LO / portable amps combos that can easily drive 650s or atleast ones with 12v DC and even with just the sflo2 HP or out will sound a fair bit better than most ordinary cans. Quality home setups don't come cheap and I will buy the best combo I can afford just becasue you dont have an amp does not mean you should not buy the sennheisers, out of interest what would you say be the minimum amp to drive 650s?
Current Cans: Ultrasone Pro 900, Denon D2000, Sen' HD650 , Ultrasone HFI-780 Brainwavs M2
Current Source & Amps: PC: Asus Xonar STX + Matrix M-Stage V2 Class A bias mod , S:flo2