There are 2 possible reasons that symbols are not previewing in the new browser.
The dll for symbol previews has not been registered. To correct this problem, close Spirit and access the main Spirit14 folder where the spirit14.exe resides using Windows Explorer - (the default folder is C: ...
The essential difference between an sm6 file and B12 is that sm6 is single precision and b12 is double precision. The precision with symbols is not as critical as with drawing files, because the origin is normal very close to the actual drawing data.
A hidden feature in Spirit is that when the enlarge factor for symbols is set to -1 symbols will be inserted mirrored. You can set this factor globally for x, y and z using the Set All option or independently for each axis.