I had a pair of Grado SR80 and found the mids much too coarse/aggressive. I haven't heard the M40x or DT990 but I own the DT880 along with several other headphones and cosy/warm/lifeless is not how I would describe them.
I don't know if you are familiar with the Sennheiser HD800S or Audio-Technica ATH-ADX5000 but the DT880 has a very similar presentation to these. Crisp, open and detailed. Linear bass that isn't bloated (some would call this lean or lacking warmth), slightly recessed upper mids, and crisply defined treble. Overall the DT880 is slightly veiled compared to the HD800S/ADX5000 but there's not much in it, and many would find the HD800S/ADX5000 too bright up top anyway.
The DT880's only flaw to my ears is a bit of treble sting at 9k that gives a somewhat steely etched sound to consonants, cymbal crashes, etc. Depending on the shape of your pinnae and sensitivity to high frequencies this may or may not bother you. Otherwise it's a fantastic headphone for the money. I personally prefer the HD600 as I find it more timbrally rich, organic, smooth and engaging through the most important frequencies (100Hz-10kHz). It's less "hifi sounding" than the three aforementioned 'phones, but I find it a smoother and more comfortable listen for longer sessions.
PS - If buying the DT880 or DT990 I'd recommend the 600Ω or 250Ω version as the 32Ω doesn't get as high praise (apparently it sounds a bit rougher and doesn't resolve as well as the higher impedance versions).
Thanks for the reply - that's very useful. I've not tried the HD800S, but the best sounding headphones I've owned to date were a pair of Sennhesier HD700. I'd love to try a pair of HD800S / 800, but I couldn't justify the cost. I did also briefly own a pair of HD600 but found them slightly a bit too dynamically laid back for my liking. I'd ideally test the DT990 and 880 next to each other and pick my favourite, but I don't know any local shop that stocks them. Tempted to just take a punt on one of them as they aren't that expensive....
Lefty