One could easily overanalyze the sound of Greenhaven. You could call them Stoner Rock, Blues-Rock, Southern Rock or say they remind you of Clutch, B.L.S. or old ZZ Top. You COULD do all that but it's easier if you call them a FUCKIN' ROCK BAND! You don't have to think to hard, just enjoy how these guy can lay it down and jam it out. They have a bad ass funk going on and some of the more original lyrics and delivery I've heard in some time. The boys "get it", they are freakin' tight as hell and write some tasty licks to go with their down and dirty playing style. Put it this way, if you like Rock and you have a pulse, you're going to at least appreciate this band.
Song by song, they do mix it up, "Flunky" is and uptempo rocker about copious drug use (or somethin along that nature). "Ape" is a laid back groover with some down-right fucking funny lyrics. Most tracks are a bit more like "Limosine", a middle of the road rocker with a nice feel to it. I would venture to say there's some Metal influences here, possibley even more into the N.W.O.B.H.M than even Sabbath (although they're an obvious influence too). They don't get so loose and fuzzy with the guitar and bass, in fact there's mixed pretty tight and compressed.
You COULD try to label this band but you'd probably fail like I did. I still think it's easier to put the fucker in and crank it loud as hell, and hopefully in the process you piss off the neighbors.