Finally got around to listening to my setup of S:flo2 -> iBasso P3+ -> Beyerdynamic DT990 600ohm. It sounds freaking awesome! Never heard a player that puts out such a clear balanced sound. I can definitely hear the difference between the HO and LO and to my ears its quite pronounced, much more than a slight difference. The LO has a much wider spacial sound stage to my ears with better detail and definition retrieval. The HO sounds a bit dull to my ears in comparison to the LO, also using the DT990 with HO produces less bass and treble is cut off whereas LO is extremely balanced. There are other aspects I can't describe really well, like the sound from LO really feels energetic, lively and punchy but the HO sounded dull and boring for me. These results are specific to my phones though as I haven't compared other phones yet. Final verdict for me is that I find the LO far ahead of the HO, I will definitely be dragging around my portable amp with the sflo2.
Another note is that FLAC is a must if using the LO, with the HO I noticed that I could use high bitrate mp3 and get away with it as the detail and flaws weren't as pronounced as the LO, but using the LO everything but Flac (and lossless in general) sounded crap. Its worth it though, the sound it produced with flac + LO was simply breathtaking for me!
I also use a flat EQ mostly but I have to say the PlayFx 3D sounds very nice to my ears but like all the other EQ adjustment profiles it can distort a bit in the very high frequencies (its mostly fine) I'll probably use it anyway as I find it improves soundstage a tad, this is the first time I have ever been impressed by a EQ enhancer thingy whatever you call em, on any player
I really love this player, best sound I've ever heard from a portable, period (from LO), I reckon the HO sounds kinda ordinary really... (only after listening to the LO) The HO is still very good and it has better detail than most daps, but it could do better in the sound stage and balance department.
Sorry I only compared HO and LO, but it simply wasn't worth comparing the sound of the sflo2 to any other player I've heard in my life as the sflo2 + LO trumps everything I've ever heard and by a fair margin.