There has already been some good information in this thread for you. Your amp has the necessary crossovers for you to run active, without the passive crossovers, but if you don't have time alignment, or any other tuning features, then running the components active is not going to give you much benefit over keeping the passive crossovers in place. You will gain the ability to level match a bit more accurately by using the gains, and you'll be able to choose your own crossover point between the mid and tweet, but that may not help much, especially if you just duplicate what the passive crossover is doing.
As mentioned, passive crossovers primarily split frequencies, not power. Although, if there are resistors in the crossovers to attenuate the tweeters, then the crossovers can do a bit of both. The signal going into the passive crossover already has significanly less power on the high frequencies than it does on the low frequencies. If you choose to run them active from the amp, make sure to set the HPF for the tweeters at a safe frequency, or you'll risk ruining them. Sending the tweeters low frequencies will be much riskier than sending them a little extra power with a proper crossover set.
Gains are easy to set. Set the gains for everything to their minimum. Slowly turn up the gains on the mids until you get roughly the volume that you're looking for with the head unit set to 75-80%. Slowly turn up the tweeter gain to get a balanced sound. Take your time with this, and if you hear distortion, back down the gains a touch. Again, make sure you set your crossovers correctly before turning the gains up. The amplifier will not automatically produce 100 watts per channel, it's power output will depend on the input signal's power, and how much of the gain you use. If the input signal is low voltage, you will need more gain on the amp than if the input voltage is high. Typical listening levels only require a handful of watts, most speakers will produce at least conversation level SPL with less than a single watt. Take your time, and make small adjustments at a time. At 75-80% of your head unit's volume range you should be pretty much at the loudest level you'll listen to, with that last 20% reserved for when you really want to crank music that was recorded at a lower SPL than most.
Having said all that, you have 2 options, keep the passive crossovers, which will ensure that the tweeter is crossed over high enough to be safe, and use the build in tweeter attenuation switch to balance the output between the mid and tweeter. Or, run them active from the amp after setting a safe crossover point for your speakers. The passive crossover offers slightly more protection for the tweeters, and running active offers slightly more tunability. Since you aren't running a DSP, running active from the amp won't give you nearly as much tunability advantage, but it would still be a bit more flexible than using the passive crossovers.
