Short answer: A good bass. If it feels good, and sounds good, and you love it, then the music that comes out, even without an amp, will be glorious.
Let me put it another way. The tone comes from your fingers, and your fingers are in direct contact with the bass. If your fingers and bass don't jibe, then it ain't happnin jack.
Long answer: I faced this question back in 1972. I had a Harmony hollow body bass playing through a Realistic (Radio Shack) amp, which was 15 watts into a cheap 15" speaker. I had been asked to join a local up-and-coming band and desperately wanted a Kustom 200 amp with the 2x15 speaker.
While I was saving money to get the amp, a friend of mine, who was in another highly regarded local band, offered to sell me his 1968 Fender Precision bass for $125. He was a cool dude, and was really trying to help me out, but, I knew I could only afford either the amp or the bass, so I ended up getting the amp, and while it was nice, and yes, I did need that extra volume for the gigs we played, I've been kicking myself ever since for not listening to my friend and grabbing that P-bass.
Oh well, live and learn.