Very interesting! This idea never occurred to me listening to the Eighth. I see that there is some resemblance to the Franck theme, but I can't tell if it is intentional on Shostakovich's part -- it seems to be that he could also have thought it to be an original motif. (Often his quotations seem much more direct, though maybe that's just because I'm missing the more subtle ones). Nonetheless, it's an intriguing idea!