What is the DT sound?
I listened to their albums in chronological order three times last week, and I was surprised that the "DT sound" is actually not the IaW and Awake sound. Some elements of the music from those albums are still heard in the later DT songs, but you can distinctly place the songs as coming from that era. I mean, insert any DT 12 song in an IaW or Awake playlist, and it would not fit.
I believe the "DT sound" that a lot of people refer to actually originated with FII, specifically two songs: Peruvian Skies and Trial of Tears. The approach of keyboard playing is different, of course, but the way they write and structure songs in general, I think, can be traced to these two songs. The way they write ballads nowadays is also more akin to the ballads in FII, especially Anna Lee, rather than ballads like Surrounded from IaW.
Not coincidentally, I believe that what people call the signature MP drum style also took a coherent shape in Trial of Tears. A lot of MP's drum fills can already be heard in Metropolis Pt. 1, but the combo of the fills, the hi-hat work, and the bass drum patterns can be heard coherently in one song in Trial of Tears. I even listened to Trial of Tears and Blind Faith back to back twice, and I smiled at hearing how similar MP's approach was in those two songs. Peruvian Skies also had the Portnoyisms (heh!), but I feel that it was just more pronounced in ToT. Of course, he solidified the style into a signature sound with Metropolis Pt. 2.
To answer the question, I believe DT does change their sound enough to stay interesting. I would be hard pressed to find an album that sounds just like another album. But I think the claim that there is a "DT sound" has merit. The "DT sound' is like combining Peruvian Skies, Trial of Tears, Anna Lee, and then a dose of The Glass Prison.
With regards to DT12, I think it should not really be expected to give us something very new in terms of soundscape. I think they have gone on record that the self-titled album is meant to express their feeling that they are now a cohesive unit. They consciously decided to be more to the point with the songs, to highlight more that they can write songs as a unit, and not rely on individual display of musical skills. I remember they even made a joke once that the role of Illumination Theory in the album is actually to assure the fans that they still are the DT that you expect, in case you are wondering after hearing a good dose of concise songs that is so not like them.