What battery are you using? What is your charging voltage, engine running, above 1500 or so? Anything between 13.8(bare minimum to keep it charged) and 14.7 is good, depending on electrical load.
Subaru engines seem to be VERY sensitive to proper clean grounding in order for the charging circuit to work well. I ran a couple extra ground straps on my wife's Subaru and haven't had a problem since. Check difference in voltage at the alternator output AND the battery. Any more than 0.1 volt difference indicates a battery cable/ground problem.
Also, since you've added all those modern-day gizmos, maybe one or more of them is drawing a little juice all the time, killing your battery softly, you know, like the song...
To test, remove ground or positive(your choice) and put an ammeter there in series between the empty post and the terminal you removed. You'll find out if there's any kind of draw/short.