The power ratings vary between the two excepting their 1 Ohm output...
D31000.1 ~
400w @ 4 Ohms
600w @ 2 Ohms
1kW @ 1 Ohm
D61000.1 ~
250w @ 4 Ohms
500w @ 2 Ohms
1kW @ 1 Ohm
Other than that one is black and the other is brushed aluminum.
I do not know if there are any differences other than that.
I just had a look at Diamond's site, though, and apparently the D61500.1 is both (a) no longer made and (b) no longer their biggest monoblock! There's a D62000.1 //content.invisioncic.com/y282845/emoticons/yumyum.gif.0556df42231b304b9c995aefd13928a8.gif