Yeah, well, I guess it's up to how define "subculture". If its just group which differentiates them from the larger culture to which they belong - I guess still remains to be considered how they differentiate? Simply based on taste of sound? Like you mentioned, if the method of consumption, the means of involvement and all that remains exactly same as for popular culture - is it really subculture?
Lets say there is someone who listens music clips from youtube. Fact that he listens Rolling Stones or Merzbow, how much does that mean if this is the end of his "cultural involvement"?
Finland is country where music culture is quite different from many other countries. Here subcultures get much bigger value than in some bigger countries. Heavy metal bands are not something what exists as minor success, but is perhaps the biggest music industry. Where punk peaked in such popular way it "tainted" pretty much entire generation. And so on.. But in that sense, it's hardly a sign of "subculture" to wear band shirt, be bald, have tattoos, or be whatever. It's all popularized as part of regular mainstream.
Yesterday I was watching piece of UK street magic guy and he appeared in backroom on popular R&B singer and next thing you see one of the crew members with Mayhem t-shirt. Few weeks ago I caught few minutes of some finnish lame reality TV or whatever show. Why? Because walking across the room with TV screen, in corner of my eye I register Graveland logo. And why would it appear on primetime tv? Well, two girls on that show had Graveland and Master's Hammer shirts. I was told not so long ago one of the C.S.I. tv series band Graveland was mentioned as part of story of involving east european extremism, hah!
But most certainly, even if subcultures bleed into mainstream, to some extent, putting out vinyl records, tapes or perhaps now even CD's, sets you apart from mainstream popular culture. However, if popular culture is basically vast variety of almost equal sized subcultures co-existing - then all of them could be seen as part of popular culture?