Great suggestions! They all mirror what I would suggest. The Peaveys are super cool, but some of the elaborate T Series wiring schemes and features haven't aged well, IMHO. They are somewhat 1970s awesome, like avocado green appliances and rust shag carpet.
FWIW, if the low/mid cut is OK on 10 and awesome from 6-8, it may be worth keeping, just for it's small, yet useful range..? It's kind of like Mesa Boogie Mark series amps, the bass control on them is only really useful from 0-4. The rest of the range is pretty much useless except under a very small percentage of situations. Accepting the T45 for what it is will be a LOT less work than what I'm about to start suggesting. It all comes down to labor vs utility. I myself love to tinker and pretty much none of my instruments are fully stock. I do like reversible mods, though.
Switching out the spin-a-split for a dedicated split switch results in much better utility: you gain full range of the tone control in single mode and a touch more air to the tone when in humbucking mode. Shunting the red wire to ground puts the pickup in split mode.
To get coil selection in split mode, a split switch and phase switch would need to be implemented since this pickup has just a single wire (red) for the series connection-which gets grounded for single mode. The phase switch would just select which coil is active in split mode since there isn't another pickup to be combined for out of phase. This would require another DPDT switch or push/pull pot.
If coil selection is not a big deal, how the pickup is wired can influence single coil selection. The stock wiring of white=hot, black=ground would activate the rear coil in split mode. Reverse the wires and you get the front coil in split mode. In a T45, the pickup is located exactly where a P bass pickup is located. In split mode, the front/neckside coil is about 3/16" closer to the bridge vs the J bass neck pickup location, the rear/bridgeside coil is 3/4" towards the neck vs a Ric bridge pickup location. After listening to both ways, I chose to have mine split to the rear coil, it sounds a touch more aggressive and punchy vs humbucking mode. Just a touch, but noticeable and more tonally/harmonically varied vs the humbucking mode.
When I built RB45 (my 'T45 Tribute' using my old, trusty Ibby RB630 as the platform), the trick was finding a 6H inductor (used for the low cut on the T45) that was small enough to put in a control cavity. Not to be found! I ended up getting a Wilde bass Q filter (3H). Did some experiments with an online RLC calculator and ear tests and landed on it being tuned to 425Hz. 400-500Hz is a favorite mid frequency zone for me on the bass, you can roll it back a little and the tone gets a clearer, hifi clarity, roll it back all the way and the tone gets really dubby and fat sounding. All the while, the low end and high end stays put. The Q filter is effectively a mid scoop control, which paired with the treble cut tone control, is a very versatile set of controls for single pickup bass.
That said, perhaps you can tune the circuit a bit to keep it effectively stock, but make it more tonally usable for the 21st Century? I do find the stock tuning on the T45 control too deep into the low mids/bass region. Adding a push/pull pot for splitting, adding a cap and maybe a resistor for the RLC circuit and a little rewire could have that T45 harness working and sounding better. This is how I wired my RB45 for mid scoop and push/pull splitting. Also tuned the treble bleed to be more subtle. Since I built this from scratch, I also used a 500K tone pot. I like the extra air it adds to the tone, air that is easily rolled off with the tone control, if not wanted. I also used a .022uF tone cap just because I only wanted to roll off the upper end of the highs. I typically use at least a .047-.068uF cap so deeper cuts start rolling off the mids. However, in this situation, I have a Q filter to do that!
For pretty similar results from your T45 circuit to sound like mine, you would use different values of caps and resistors on your RLC: a .022uF cap and 10K resistor with the stock 6H inductor tunes it around 440Hz with the same Q factor. I tend to mostly play the bass with everything on 10 and humbucking mode, though I do split into single mode here and there and also sometimes roll the tone controls back a touch, as needed. It's crazy versatile and looks clean and stock.
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