Regarding the relationship between sukha and dukkha, Rhys Davids & Stede (1921-5), p. 716, entry for "Sukha" (retrieved from http://dsal.uchicago.edu/cgi-bin/philologic/getobject.pl?c.4:1:262.pali), simply identifies that dukkha is one of the antonyms of sukha (along with asukha) and, in such contexts, is sometimes spelled dukha.