The vowel sounds in "près" and "pré" are very clearly different, and the sound in "prêt" changes from "è" to "é" when in liaison because it always sounds like "è" at the end of words (and separately, in closed syllables) and always sounds like "é" in open syllables otherwise (liaison triggers a change in the syllable structure which changes the vowel here). This does not contradict what I said. You said "(pr)é" and "(pr)è" sound the same, nothing about "(pr)ê".