Hi all
@mrAdrian - as we have similar setups, thought I'd toss my 'two-pennies' in .......
Your setup - NFB 5.2 -> DV336 -> HD580, K400, D5000, MS Pro
Mine - NFB-12 -> LD MKIV -> HD600, K701, DT880, SR3251 (woodied)
My aim was similar to yours - rather than continually climbing the tree, I wanted to try most of the former flags (or close to them), and have variety that would totally satisfy me. I do find with the current range I have, I've ceased looking for gear now, and am far more interested in the pursuit of music. In that regard - yes, I think the DT880 does serve nicely to round out the collection. I personally find they're a great all-rounder (particularly with tubes), and just give a slight smile rather than a real V. I know a lot of people regard them as being slow - but it's something I've never found - mind you a lot of my music is slower paced (prog and classic rock, jazz, classical, acoustic, indie and folk). I'm not really into too much really fast paced stuff.
The one thing I do like about the DT880 is the imaging. The SS is by no means huge - but I do find it pretty accurate. The K701 can sometimes be too big (SS), whilst the HD600 can sometimes be a little too laid back (I love it - but sometimes I crave a little more sparkle).
For me, it bridges the gap between the K701 and the HD600. And then of course there is the 325i - which is just flat-out fun
Anyway - all 4 cans get pretty much equal time - just depends on what I'm listening to and what my mood is.