A Blend that actually Blends

The main thing you're experiencing is just badly-chosen pots. Manufacturers too often just use the parts that are cheapest, even if they are not the ideal spec, and pots are often at the bottom of the barrel in terms of company priorities.

Interestingly, this is not just true for passive basses, but also most active basses because the majority of them just have a passive blend stuck onto their active circuit, so you get all the flaws of a passive system just in the blend function.

There are a few active blending preamps out there--Audere and Glock are the two I can think of off the top of my head--but they are a small minority.

So if you have a passive bass, the solution is to remove the existing blend circuit and replace it with a better quality pot that has a more useful taper on both sides (because a blend pot is really two separate devices controlled by one knob).

If you have an active bass, the same advice is true if the blend in it is passive. If you have one of the less common active blends, then it gets a lot more tricky, and your best bet would actually be to contact the manufacturer for advice.

 

I've been on the same search, here's the two I've found:
The Alps brand
http://www.guitarelectronics.com/pr..._Guitar_and_Bass_Pickup_BalanceBlend_Pot.html
Alpha brand
http://www.darrenriley.com/shop/cart.php?m=product_detail&p=1435

I don't know if these are good or not though, I'd really like to find a nice accurate blend pot to put in my Jag bass and then use a concentric pot for the Vol/Tone

 

Alps is generally quite good; Alpha is "OK".