There was an earlier post in the thread by a TBer named Duffy which contained a lot of misinformation that I feel I need to clear up.
First off, switching to flats for the purpose of getting out of a guitarist's domain is pointless. The vast majority of "cutting through guitard" problems come from them stepping into OUR territory by cranking too much bass and low-mid into their sound. You'd be much more successful cutting the bass on the guitarist's amp than trying to reduce the high-frequency information in your signal.
I'll agree with the large diameter strings putting out a larger signal (more mass = more change in the magnetic field over the pickup), but you actually get more of the fundamental with low-tension strings. Ever notice how downtuning sounds MUCH deeper when the strings are slightly too loose to handle the tuning? The first few harmonics (octave up, octave + 5th, 2 octave) are slightly de-emphasized when a slacker string is used, in my experience.
Elevation is great, but don't just do it for the bass amp. The guitar amp needs to be pointed at ears, too.
On the issue of speakers sensitivity vs. wattage, doubling your cone count (assuming identical cabinets) is a 3dB bump in sound pressure. Doubling your wattage (assuming you're not bottoming out on your current speakers) is also a 3dB bump in sound pressure. Using a second identical speaker on a solidstate amp will therefore produce a 6dB bump, because the added cabinet will cut the impedance the amp sees in half (ie: roughly double the power). None of these figures is a doubling of volume. To get twice as much volume, you need a 10dB boost. That means 10 times as much power, or about 8.5 times as many cabinets of equal sensitivity, all within coupling distance of eachother. Fortunately, you rarely need to double your volume to compete with the band.
While it is EXTREMELY true that a more sensitive speaker could provide a lot more volume than the stock speaker in a lower-end combo, it's also just as likely that it will sound like trash or possibly destroy the speaker at what used to be moderate volumes. A ported cabinet MUST be tuned to the speaker that's in it, and more often than not, the tuning required for the stock speaker in a combo won't be remotely close to the tuning required for a higher-end speaker.
Finally, the issue of hot pickups and boosting gain before the preamp... This is a futile effort at best. Unless the preamp can't reach levels suitable for driving the poweramp on its own, boosting the signal before the pre just means you can run the gain knob lower. It might SEEM like you have more volume on tap, but that's only true if you dial with your eyes. You'll still hit the wall in terms of volume at the same point, and it will seem like you hit it faster because you have the gain knob set lower to reach the same level as you did without the hot pickups or signal booster.
The truth of the matter is, OP needs to learn how to work with what he has. EQ and amp placement are the only choices he has outside of adding more speakers or replacing his rig outright.
EDIT: No offense meant to fellow TBer Duffy. I'm sure he's an absolutely stand-up dude and is genuinely trying to be helpful, but even the most astute of us can be fooled when myths get propagated as truth for long enough.