These first two weeks in December are for the more active among us — well, musically at least. EXPLORING MUSIC (1000) ascends some of the most colossal musical mountains in existence, works like Schoenberg’s Gurrelieder and Havergal Brian’s Gothic Symphony, that are (usually) too big to program on an hour-long program. The following week, Bill McGlaughlin goes prospecting for hidden gold — “some works that are absolutely fantastic, as fantastic as the pieces we hear all the time, but not played as often as the pieces we always hear.”