Well for me the term basshead headphones just means it has a quantity of bass that is north of that of +5dB or so. There are good sounding basshead headphones and bad sounding basshead headphones.
Good sounding basshead headphones (at their pricepoint) is for example Sony XB series, AKG K518, K81, K181, Audio Technica ATH-WS55, PRO700MK2, Ultrasone HFI-580, V-Moda, LP1, LP2, M100, M-Audio Q40, UE6000 etc.
Bad sounding basshead headphones would be for example the old lower-end Skullcandy headphones, Beats by Dr Dre headphones.
Quality wise you should be listening for if the bass sounds controlled, is it boomy (single note bass without any detail, compare car subwoofer setups, it's just bunch of WROOM-WROOM, BOOOM-BOOOM sound) or textured (all bass details are present)? does it affect mids or not? Does it only appear where it should be? Is there a lot of resonance (which will result in lack of texture as the finer details in bass will be masked by the reverb and sounds "mushy"/smeared in the bass)? Is it loose (slow, a bit all over the place) or tight (punchy, snappier, this kind of bass usually lets you hear more details)?
Of course all this has very little meaning to you if you haven't heard several different headphones so you get a little better experience of exactly how it can sound like, even a bad quality basshead headphone can sound very good to you if you haven't heard any better and don't know how it CAN sound like. At first I was using a Sennheiser HD212pro which is certainly a basshead headphone and I absolutely loved it back then. Today I'm facepalming when I put them on and can't understand how I could concider that headphone sounding so good.