I took the "ugly" ePub converted from KFX > ePub (and even tried it KFX > AZW3 > ePub), attached it to an eMail, sent it to my Mac, and opened it using Calibre's ePub reader. Looks fine. So could this simply be a case of a font missing from my PC?
I've uploaded the ePub to a cloud share so anyone with the inclination to look at the ePub's css (and the knowledge to interpret the results) might be able to determine what's going on here. The body copy at issue is "class10" and "class11" - both spec'd as "Roman" font. They look identical in the ePub (just fine on my Mac but ugly on my PC).
http://cloud2.zoolz.com/s-66YClaZr