Been there, checked it out. Basically, the stock ATK pickup is in the traditional MM position, which means that it half-covers the traditional P position.
Your options are therefore limited:
1. You can install a P-bass pickup at least a half-pickup-width closer to the neck than normal.
2. You can make room for the P-bass pickup in the traditional position by enlarging the stock pickup route, and moving the ATK pickup back towards the bridge a bit.
The second option is fairly involved.
First, there's the bridge. The lip that surrounds the pickup on the ATK bridge plate is in the way. So you'd either have to convert to a conventional bridge, or cut the leading edge of the existing bridge plate. I came up with an alternate pickguard shape that would work in that application- there's a pic on the two pickup thread.
Second, there's the pickguard and existing pickup rout. With the ATK pickup moved out of the way, the new P-bass pickup would be sitting half over an open bathtub rout. One fix for this would be to extend the pickup route a little further forward, and use a P-bass voiced soapbar pickup of EMG 40-series dimensions. Another would be to simply bang a regular P-bass split pickup in there, and cover the unsightly gaps with a new pickguard.
The consensus is, that the ATK sound would be harmed by moving the stock pickup closer to the bridge. So option #1 probably makes far more sense. The P-bass pickup would sound a little fatter than normal in its new position, but that's not the end of the world, since you have the ATK pickup to provide some zing when needed.
Electronically, you'd have a couple of options:
1. Precision p'up wired to a stock P-bass wiring harness, with a two-position switch right before the output jack, to select between passive P and active ATK operation. No blends possible, and there's likely be a healthy jump in level between the two modes. There's room in the stock cavity for the extra controls.
1a. Ditto, but both sides active, with some sort of booster preamp to match levels (I've used the EMG PA-2 booster in the past, with decent results).
2. Precision pickup wired to the input of the ATK preamp through a 3-way switch, Gibson style. No individual volume controls for the pickups, blend fixed, and controlled by pickup height.
3. Add a vol/vol wiring harness or a blend pot between the two pickups and the input to the ATK preamp.
Based on my experiments so far, options 2 and 3 will have some unusual interactions between the pickups, because the second pickup "sees" the filter contained in the ATK character switch. The blend sounds good on some character switch positions, not so good on others, and the result is dependent on how the pickup heights are set.
This is why I'm intending to try out a 4th option: inserting the new EMG ABC active blend pot between the pickups and the ATK preamp. It has an active buffer on each input, so the pickups would be isolated from each other.
In the end, my two-pickup experiment took a different tack. With Jamie Funk's help, I dropped a second ATK pickup into the "woofer" pickup position, like on an old Gibson bass, or Billy Sheehan's Yamahas. Easy to do, because the new rout is covered by the stock pickguard...
The experiment continues (stalled at the moment by a lack of funds), but early indications are that twin ATK pickups in the woofer and Ray positions sound pretty good, but not P-bassish at all.
If I was doing it over, I might have tried a Warwick double-buck setup- Two ATK pickups, set very close together, wired to an EMG blend pot.
Anyway, for pictures, etc., of my two pickup experiment, go to this thread:
http://www.talkbass.com/forum/f38/716577-atk305-second-pickup-options.html
I dont do much tweaking, im not much of a knob twiddler. I almost always play in attack mode, flat. on one of my atks, the one set up for tapping, I set it to bright mode, and boost the treble some. sometimes on slower songs Ill hit on the body of the bass lightly repeatedly to get the notes versus plucking the string, and Ill boost bass to get more body out of it, similar to a pad on a keyboard. thats just me though.
