I have an XT-25, 5 string Spirit. I have upgraded it with soapbar pickups, a three band preamp, 18 volts. I'm still not happy with the tone, so I replaced the strings, using GHS double ball ends. Not too bad, more output but not much happening in the high end.
I had a Kramer headless bass, short scale, aluminum neck...the Duke. I loved it. I replaced the pickup with an EMG, but the stock Shaller sounded better. I added a second Shaller. Great bass.
I switched to 5 string in the late 80's. I got the Spirit 5 in a trade for an amp. It got damaged in a car accident a little over a year ago, so I'm getting it back in shape. I did a lot of experimenting on the strap positions, and I'm happy with where I mounted the strap buttons. I'll add pics later.
Let me say something about the EMG Selects used in the 4 strings. I have used EMG Selects in guitars over the years. They usually sound a bit dull and midrangey. The tone benefits GREATLY by simply adding a buffer. The tone comes to life. Adding nearly ANY preamp will help a lot.