I bought a used Martin D-15 off ebay and while it plays and sounds amazing, the intonation is terrible. I'm not sure how I didn't notice it right away, but essentially, you can have all the chords sound good enough except an open E, or the E sounds right and everything else is sour. There was a crack in the neck around the first fret, but it was repaired and looks & feels fine. The intonation up the guitar neck is fine, but the low E, G & B strings are all sharp on the first 2 frets. I brought the guitar to my local shop and asked them to lower the action a bit and fix the intonation. Well, they dropped the action way down and as far as I can tell, didn't do anything to the intonation. I have practically no experience with an acoustic. Can this typically be fixed with a new nut & saddle, or am I sinking money into a guitar that's not worth it?