SOHW-1673S and SOHW-1693S [and ONLY those two drives from the 3S series] are the same hardware, and can run the same firmware. If there is a hardware difference, it makes no noticeable impact in write quality and has never been discovered. People upgraded their 1673S drives to 1693S drives without a hitch. The only change between the two, feature-wise, is that the 93S supports writing DVD-R DL [not that widespread outside of Asia, if I remember correctly].
Other drives from the 3S series [1633S, 1653S, and earlier] could be crossflashed within each other, but a hardware change was implemented in the 73S and 93S that prevented this.