In traditional British RP pronunciation, the adjective comparable (and its opposite incomparable) are stressed on the syllable '[-]com'. Despite the derivation from the verb 'to compare' ('com-PAIR', IPA: /kəm 'pɛər/), there is no regular pronunciation
'com-PAIR-able' (IPA: /kəm 'pɛə rəbəl/. However, LPD (2000) shows a tendency for younger speakers to use this more and more. In GA, it was the majority realization in a 1993 poll; in Britain, it was preferred by approaching 40% of younger speakers in 1998.
Say 'COM-per-ebl', /'kɒm pə rəbəl/ and 'in-COM-per-ebl', /ɪn 'kɒm pə rəbəl/.