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Fender Ultra P + J also have compound radius boards (10" to 14"). Plenty of other basses out there with compound boards.

Are you sure the pots are 473k? The pic above shows the number 473 on the tone cap which signifies a 47nf cap (a standard value). Or is that just a coincidence?!

The Delano pickups should have plenty of top end. What are the pot values? If they're 250k you could try changing them to 500k which would brighten up the sound.

Just sold my Boss RC-3 looper to Russ. Top chap, quick to respond to messages and a pleasure to deal with. Happy looping!

No, tightening the truss rod gives you less relief, slackening it off gives you more relief, see diagram below.

How it affects the solo'd P sound depends on how it's wired. If a P/J is wired as a regular VVT then the additional vol pot will change the load on the P pickup which does affect the tone. In the same way that swapping 250k pots for 500k makes the sound brighter, adding an extra pot (in parallel) reduces the load and makes the sound a little less bright. Soloing the P pickup in a VVT setup will sound similar to running a standard P with the volume backed off a bit. A smidge less grunt. You might not notice it unless you really listen for it but it does make a difference.

This TB thread is about a similar problem - https://www.talkbass.com/threads/peavey-millennium-bxp-ac-preamp-cuts-out-when-treble-control-turned.1098452/ What make of bass is yours? As a first step it might be worth giving the iffy pot a squirt of contact cleaner to see if that makes any difference.

It would also be possible to add a switch to completely remove the added J pickup (and vol pot if it has one) from the circuit allowing you to switch between a P/J config or the original unmolested P config.

This article on fixing a B2A with dodgy wiring contains a wiring diagram which might be useful - https://richsstudioproject.wpcomstaging.com/2015/08/19/hohner-b2a-active-electronics-repair/

Crikey, that's a bit of jumble!

Have you tried tweaking the trim pots for the mid control in the control cavity? You can adjust the mid freq and the level of cut/boost if you're not happy with the factory settings. Might be worth having a play with that before replacing the preamp.

If it doesn't work in active mode, even after changing the battery, then just switch it to passive and use it in passive mode. To connect it to a software package like GarageBand you will need an audio interface like a Focusrite Scarlett or something similar - https://www.amazon.co.uk/Focusrite-Scarlett-Solo-Audio-Interface/dp/B07QR6Z1JB

Here's the Ric wiring. I've tweaked it to take the switch out of circuit. The 4.7nf cap in the bridge pickup signal path cuts some bass which gives the characteristic Ric clank. Just remove it if you don't want that. Again you might want to change the pots to 500k for your Spector humbuckers. This circuit uses a stereo jack but you could use mono and just connect both hot wires to tip. It has no isolating resistors so there will most likely be some interaction between the neck and bridge pickup controls. Standard Ric wiring though so I guess it works!

I believe they're there to stop the tone controls interacting with each other. If you remove them, the hot wire from each pickup has a clear path to both tone pots. A side effect is that they also attenuate the signal somewhat which isn't ideal. I'll see if I can dig up a schematic for a Ric as an alternative.

Jazz stack-knob VTVT wiring is essentially the same, just use 4 separate pots rather than 2 stacked. If the Spector has humbuckers you might want to make the vol pots 500k instead of 250k. See diagram below. Tone is on top, vol underneath.

Depends whether the piezo element is bonded into the saddle or just a push fit. I have a Hipshot piezo bridge and the elements just lift out (pic below). If the hole where the wire comes out is large enough, I'd try giving it a gentle prod with a stiff bit of wire like a paper clip.

The TC Electronic RH750 head has a built in tuner but no aux power supply. https://www.tcelectronic.com/Categories/Tcelectronic/Bass/Head-Amplifiers/RH750/p/P0CIV#googtrans(en|en)

Looking more closely at the El Torro active circuit (see both links below), I think, if you ignore the active/passive switch (S1) and the preamp, what remains is the wiring for the original passive version. The preamp appears to just be a signal boost to present a stronger and cleaner signal to the amp. The treble and bass controls are provided by a passive circuit. Active circuit schematic - https://www.bassesbyleo.com/images/el_toro/tech/el_toro_wiring_diagram_drawing.pdf Active circuit description - https://www.bassesbyleo.com/images/el_toro/tech/el_toro_wiring_diagram_description.pdf The passive wiring would then look like the schematic below. See the description link above for the function of S2 and S3.

This passive L-series wiring diagram may provide some clues - https://www.bassesbyleo.com/passive_l_series.html

The Stingray pup position actually overlaps a 60s J bridge pup to quite a degree. It might just about squeeze in with a 70s position J-style bridge pup without mounting lugs. Here's a comparison pic.

Exactly. Mine purrs like a contented cat - an original '57 (with Clive Brown refin). It just makes me smile 😊

Ah yes front ported, don't know why I thought sealed. And thanks, if we ever do!