Positional play: Slav the way to go? I'm working on my positional play. Exclusively playing correspondence chess at the moment.

For white I play D4 and try for C4 after that.

For Black I play Caro Kann against E4 but am not sure what's most positional against D4.

I'm thinking of Slav, Semi Slav, QGD

For solid long term positional play which one is best investing time in learning in depth?
I recommend to play Stonewall Dutch, quite closed, yet quite active. 

[Event "Stonewall Dutch"]
[Site ""]
[Date ""]
[Round ""]
[White ""]
[Black ""]
[Result ""]
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

1. d4 f5 2. c4 Nf6 3. g3 e6 4. Bg2 Be7 5. Nf3 O-O 6. O-O d5 7. Nc3 c6