137 is obviously a number very important to the Grand Unified Theory. So everything is connected. D/T is obviously the sequel to Octavarium, but it's also the sequel to SFAM, and the sequel to TA, and the sequel to SDOIT. It connects all the previous 136 songs from DT in a grand scheme of interconnected storylines. It's the most ambitious work yet, and so cleverly put together.
And I mean, it's obvious, isn't it... 1 3 7. 5 8. 2. 6. Get it?
137 from D/T. 5/8 from Octavarium. 2 from metropolist pt. 2. 6 from Six Degrees. 4 as in the fourth album with Mangini. It all makes sense!