I have a pair of K271 Mk.I that I use at work (with velour pads, so should be quite close if not identical to the Mk. IIs). I have a pair of 840s at home. I dont do reviews, etc. and don't usually talk much about my impressions of things, but my experience would say that the 271s have less bass (though adequate IMO), and are more neutral than the 840s are. The 840s have a mid bass peak, and while "fun" phones to listen to, are more colored.
Both of these driven with Gilmore Dynalos.