Wow! Thanks for all the advice everybody! I really appreciate it. I do have a few more questions though, related to the advice you guys have given me.
1. On replacing the Acoustic with a Fender Rumble 500 combo - isn't this generally not advised? Mixing speaker sizes I mean. It should be different because it's a combo, no?
2. Even though the school is providing an upright, I'm thinking that getting my own would be nice. Where would I start?
3. @static0verdrive I've thought about a fretless before, but would it bridge the gap between an upright and an electric? There is a fretless ESP PJ bass for $350~ at my local music store, I really like it, but would it be worth it?
4. About replacing my J pup, would I have to match the impedence? My P pup is a Tonerider P vintage P pup rated at 10.6k (fantastic pup BTW). I've found that the GFS overwound J is closest at 10k.
To clear up some stuff:
I did love playing with the six string bass, but my problem is that the only time I use the B string is when we played Uptown Funk (our finale). I never used the C string. The only reason I used the 6 string was because it was set-up for a low action with roundwounds - perfect for the slap-and-pop parts of Uptown Funk and for nailing that tone. Otherwise, I use my PJ minus the J for other songs, it's set-up with heavy flatwounds with a nice mid-height action.
@Matthew_84, He's the reason why I picked up the bass

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Also, the book we're using in the HS Jazz Band is the same one I'll be using in college. They actually provided them! We'll be using the same next year (in my freshman year). I've looked through the entire book, and never did I see any note below the standard tuning of EADG. The highest I've seen that I'd have to go is a D on the seventh fret of the G string. As of now, I don't really see the use of a six string, even though they are nice!
