Having been experimenting with P90s in a couple of guitars I'd like to share with you a couple of things I've discovered along the way, which may help you improve the tone of your instrument or maybe help you out of a tone-problem issue. Good P90 pickups (we're talking the likes of Lollar, Fralin, Haeussel, Harmonic Design) - both "soapbars" (the rectangular ones that go into the p.up route) as well as "dogears" (the ones that have the 2 triangular "ears" on the sides) - are single-coil pickups. This implies a given amount of noise (RI, 60 cycle-hum, etc) but on the other hand there is a sensitivity to these p.ups that can't usually be matched by a humbucker. Tone is often described as brash, spanky, gritty, bluesy, in-your-face, etc and for certain genres a good P90 just can't be beat. However there may be situations where that elusive tone isn't there - let's see what we should know:

• Pickup height: These pickups can be relatively close to the strings, if yours sound lifeless, try jacking it up
• Screws: Major tone factor, here - raise the screws for a snappier, strat-like tone, screw them all the way in for a smoother response
• Pots: P90s are single coils, hence you should use 250KOhm potentiometers...but experimenting can yield interesting results!!
• Middle Position: The tone which results from the combination of the 2 pickups connected in parallel is a result of polepiece distance between the 2 p.ups, and the phase cancellation that occurs from the fact that the polepieces are sensing 2 different parts of the string's vibrating pattern at the same time. I found that the most interesting and pleasing tones result from having two radically different base tones from your neck and bridge pickup. The phase cancellation will eliminate the frequencies that are in common between both pickups. In my experience the longer scaled instruments seem to have a warmer-toned mid position, while the short scale guitars have a thinner sounding mid position.
• Serial *and* Parallel Switching: install a push pull pot in order to combine the neck and bridge pickup in series - you'll end up with a big humbucker sound that could actually be quite useful in some circumstances!
• In use: Good P90s are very sensitive to slight roll-offs on the tone and/or the volume pots. It could easily be that your dream tone is t be found on "8" and not on "11" ... ;-)
• Don't Forget The Trussrod: A slightly relieved (="curved") neck has a warmer tone than a perfectly straight one. On one hand there is less tension in the rod and the woods - which affects the overall resonance of the neck - and on the other one, the strings are going to be fractionally higher from the frets and therefore there will be less harmonics being generated by the string vibrating against the immediately adjacent fret (1 away from your finger towards the bridge)