Sibilance would be like a diffuse/fuzzy "spike" of highs/treble frequencies on the "S" "F" and sometimes "T" and "D" sounds on vocals (that is my noobish interpretation). It seems to be "naturally" present on nearly every recordings but especially noticeable on poorly masterized, or poorly encoded (eg low bitrates mp3) ones.
It also seems that sibilance sensitivity could vary from people to people.
I dont own the s4 but own the hippo VBs myself and based on what i read i think both probably have a pretty similar sound signature : somehow warm and bassy with extended lower end, good mids, and somehow brightish with extended, somewhat harsh treble. I dont know of anybody who owns both but i havent concluded that the VBs would be more bassy and/or less clear than the S4. Actually it seems to me that the VBs would extend lower with slightly less midbass hump than the S4, with maybe a more neutral midrange and a similar extended and not-so-refined treble.
I can affirm that the VBs bass indeed extend audibly to impressively low frequencies but is also free of any midbass-hump suspicion. (i would actually probably enjoy a slightly higher midbass on my VBs even though i am not really a basshead)
I do have a problem with its treble though and would say that if you are, like myself, very sibilance-sensitive you should stay away from any phone that is suspect of "brightness", "treble harshness" and even slightly above average sibilance.
I havent heard the RE0 yet or any other phone with a reputation of extended AND very refined treble, but it seems to me that any phone that can be qualified as "bright", "treble-centric", "hugely detailed treble" would only exacerbate any sibilance already present in the recordings and make it unenjoyable to highly sibilance-sensitive folks.
But then, the magic of burn-in (be it in your brain or on the actual mechanics of the phones) can change things : i am now enjoying my (foam modded) VBs a LOT even though i still get bothered by sibilance from time to time.
I can easily see the VBs among the overall best IEMs you can get for under 100$ for anybody that is not overly sibilance-sensitive and naturally inclined to like darker sound signatures.
EDIT:
other contenders for your needs : earthsounds thunder, both cyclones pr1 pro (not sure about the bass, have yet to receive them) and pr2, radiopaq jazz, meelec ai-m6, audio-technica cks70, FA eterna (maybe too far from neutral coming from pl50) and silver bullet (not sure about the bass quantity), crossroads woody 2, monster turbines (if you can get them under 100$)
EDIT2:
i suddendly realized that you probably shouldnt pay attention to my comments about VBs or S4 or whatever sibilance, since you are very probably NOT highly sibilance-sensitive. The first time i stepped on the word "sibilance" (it was on these forums) it made immediately and intuitively sense all by itself to me (SSSSiblianSSe!) without me having to look or ask for a definition, because i had very often noticed or been bothered by this phenomenon to some degree before. If you were highly sibilance-sensitive yourself, it would also have made sense to you immediately.
