That Highway One tone circuit is the Fender "GREASEBUCKET" tone circuit. It "lets you roll off treble without boosting the bass." So the lows you have are all you get no matter what you do with the tone control. It is a feature of both Highway One series guitars and basses. I’m not partial to any tone circuit with a resistor in it personally because I feel anytime you put a resistor in a tone circuit something is getting reduced and I want ALL the juice!
Most Jazz circuits only have one cap. The GreaseBucket circuits on all the Highway One model is one exception. The other exception is coming up.
If you want a stock one-cap Jazz Bass type Neck Volume/Bridge Volume/Tone circuit the JACO circuit works with one cap here:
You can do that one.
Personally, on two pickup basses I like a MASTER VOLUME with PICKUP BLEND and one MASTER TONE. I want to be able to turn up one volume and be ready to go or kill that one pot and be dead quiet. You can't do that with a typical J-bass circuit which was always a major stock J-Bass gripe for me and one of several reasons I prefer P-basses.
If you replace the BRIDGE pickup pot with a stacked blend pot you can use the schematic at the following link and have a Vol/Blend/Tone setup with one cap.
You can get the blend pot required in that setup from about any online guitar parts source. You can use your existing pickups, upgrade to a PIO cap and get out of the mod for under $30.
The other exception to the "One-Cap J-Bass Circuit" is the Bill Lawrence circuit for his revolutionary 3 lead J-pickups which call for a treble bleed cap on the BLEND pot in addition to the tone pot cap. Bill Lawrence will make you a custom 3 lead J-bass pickup set that uses your existing single pole pots to get a Master Volume/Pickup Blend/Master Tone setup if you ask him for them. The guy is a GENIUS when it comes to passive pickups and tone circuits….among other things. His pickups and control circuit use two caps also, but no resistor and would be a similar circuit to his 3-lead P/J setup I’m running in a P/J bass which is illustrated here:
In this circuit I settled on a 0.0.18 PIO Sprague Vitamin-Q type cap in the variable position to get the most treble out of it. I used a 0.047 PIO Sprague Vitamin-Q type cap where he called for a 0.050. I should point out that while this circuit is drawn Blend/Volume/Tone, you can move the Volume pot to the top position and the circuit works perfectly, which is precisely what I did.
So if I was going to overhaul the tone circuit on a Highway One, I’d go all the way to a Master Volume/Pickup Blend/Master Tone arrangement of some sort and toss the GreaseBucket circuit. But, that is JUST ME. I had a Highway One Jazz for a couple of months. It had a neck issue and I eBayed it as broken before I had a chance to mod it. I planned to mod it because having two separate volumes was just not me. However I didn't get the chance.
Highway One basses will never be truly collectible, so modding them is perfectly okey dokey. If you've got a stock 1965 or previous J-Bass do not even think of any control mods please. Leave it stock.