I don't see how the voiced bilabial stop, /b/, is any easier to pronounce than its velar /g/ or dental /d/ cousins. Phonæsthesia (sometimes sound symbolism) is problematic at best for me. It seems to be closely coupled with a particular language's phonology. Think of nasals in English, /m/ (written hmmm) is positive though it's also a bilabial. And not all nasals are positive: e.g., /N/ (the final consonant in sing) sounds slightly negative to my anglophone ear. Take the vowels in English: the lax back vowels /a/, /o/, /u/ sound positive, but their tense counterparts /O/, /U/. (The lax/tense contrast is sometimes called open/closed in European linguistics.) Compare front lax vowels: /i/ and /e/ versus their counterparts /I/, /E/, and perhaps /æ/. Diphthongs and triphthongs bring a whole 'nother dimension into the mix.