Well the KvR forum has over 100k members, when I checked a year ago, spanning from regular bedroom producers John / Jane Doe's to professional studio musicians with many years of experience. It has a forum called Music café that lets you post your music and get reviews from the members there. And since there's pro musicians with decent knowledge about marketing I'm sure you can find something useful there.
Now I dunno what's up with KvR at the moment. It seams their site has some issues currently. But I'm pretty sure they upgrade something on their site, so check back there during this weekend.