This may be way outside of your wheelhouse, but I have to ask -
The Mandarin reading of 源 is yuán. The phonetics are something like [ʲyɛn]. A similar raising of the written vowel occurs in the pinyin syllable yan (e.g. 言, 严, 眼), which is [jɛn]. In other pinyin syllables, an "a" represents /a/, which is something sort of intermediate between the English PALM and TRAP vowels.
It makes perfect theoretical sense that /a/ might be realized as [ɛ] when following a high vowel. But I've always wondered whether yuan and yan really do have a phonemic /a/ there or whether there might be an /ɛ/ phoneme. And it's interesting to me that the Japanese reading of 源, presumably taken from a much older Chinese, uses /e/ there instead of /a/. Can you provide any insight?