Are you not getting any sharp pics for the above combo? I have one myself and the pics are relatively sharp. Try to Step down to atleast one stop, like f/8. And ofcourse use tripod if possible or fast shutter speed if you are handholding.
50mm f/1.8 has a MkI (metal construct and mount) and a MkII (all plastic except for optics and circuitry) version.
50mm f/1.4 USM only has one version.
Personal experience with the f/1.8 MkII is that it's sharp from f/2.5 to f/8. Never shot smaller than f/8 because I use it in low to moderate light the most.
have used it with the kenko pro 1.4x before. results seems ok wide open but need to step down to increase sharpness. remember the shutter speeds though. if it's not fast enough, you tend to have camera shake which will cause blur pics.