I've mistakenly used /ə/ rather than /e/ in essayer, professeur, essentiel. Is "ess" always /es/?
Either /e/ or /ɛ/ but very rarely /ə/ indeed. This is true for every E located before any double consonant. Despite the accented pronunciation, there is never an actual acute, grave or circumflex accent on these E's, no exception.
On the other hand, there are a few words of this kind where the E is pronounced /ə/ like cresson in some regions.
A larger group of exceptions is composed of words having the prefixes de- or re- and where the double consonant is only there to keep the pronunciation of the unprefixed root:
- dessus (a hypothetical desus would tend to be pronounced /dəzy/)
- ressortir ...
- Montpellier (except locally where it is pronounced /mɔ̃pelje/)
- ardennais (locally)