I will try to answer some of your questions, but hopes others will chime in. I don't have the 1770 anymore I replaced them with the 1990. For me the 1770 was a little bit too closed and lacked air. But I would not call them thin or dull and they are very good for rock/metal music. But for my taste the 1990 did the job slightly better.
The T5p.2 are thinner sounding than 1770 and 1990 but brighter, more detailed and bigger soundstage and more expensive....
If I had a limited budget and could only have one it would be the DT 1990.
The dp5 is very easy to drive you can literally drive it with a smartphone or a decent dap. 1770 and 1990 are also relative easy to drive. I use my mojo and my LG V10. It actually sound very good of my V10.
The T5p.2 are thinner sounding than 1770 and 1990 but brighter, more detailed and bigger soundstage and more expensive....
If I had a limited budget and could only have one it would be the DT 1990.
The dp5 is very easy to drive you can literally drive it with a smartphone or a decent dap. 1770 and 1990 are also relative easy to drive. I use my mojo and my LG V10. It actually sound very good of my V10.