Provided herein are compounds of Formula (I): and pharmaceutically acceptable salts thereof, and pharmaceutical compositions thereof; wherein X, R1, R2a, R2b, R2c, R2d, are as defined herein, and Ring HET is an optionally substituted 6,5-bicyclic heteroaryl ring system comprising 2 to 5 nitrogen atoms, inclusive, wherein the point of attachment is provided on the 6-membered ring of the 6,5-bicyclic heteroaryl ring system, and wherein the 6-membered ring is further substituted with a group of formula -L1-R3, wherein L1 and R3 are as defined herein. Compounds of the present invention are useful for inhibiting CARM1 activity. Methods of using the compounds for treating CARM1-mediated disorders are also described.