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I have a few Dr. Z amps, but don't have a Jetta, so i can't help you directly, except to recommend that you look on Z-Talk and The Gear Page. This amp has been discussed a lot on both of those forums.
The Z amps that I do have- a Cure, a Nova and a Z Wreck Jr, are awesome.
I just received a coupe of sets of Ernie Ball M-Steel Hybrid Slinky Strings (.009-.046) for my Music Man Cutlass. I wanted to hear how it sounded with the strings it came with as kind of a baseline. They sound okay but there's a lot of finger noise. We'll see how they play in.
I'd contact Lollar and ask, though thier baseplate might have the narrower spacing. I had another vendor make me a P90 like that to but in the bridge position of a '63 Epiphone Olympic due to it requiring a low profile pickup.
Replace the electrolytics for sure. F&T are good. Yellow Astron coupling caps get leaky. Keep them in the amp if possible and replace them as required. Possibly consider replacing them between the phase inverter and power tubes to prevent issues. I like to replace B+ dropping resistors as well.