[Weigel21]

Bass boost is typically shunned, as it's VERY easy to induce distortion or even clipping. See, for every 3dB of boost, an amp is asked to double it's power output.

Frequency response specs are always iffy, unless there's a response chart with it. See, the CT's may play down to 50Hz (looks like 55Hz to me on their website), but how well is unknown. Typically, the frequency response specs assume a +/-3dB variance in the output throughout the response range. I'll go ahead and stop here on frequency response, as it can get quite confusing, and I'm horrible at explaining things.

Still, I'd not cross the speakers over as low as you are asking, in fact, I'd probably not go lower than 80Hz, unless you are relying solely upon the CDR's for your lows. Still, chances are they won't have a lot of oomph below 80Hz or so.

Now, using two crossovers together is another thing typically shunned. So use the crossover of the amp or HU, but not both together.

Your HU's EQ will definitely affect the output, even with an external amp. The frequency is obviously the frequency greatest affected by the boost/cut. Level is the boost/cut. And Q is how wide the effect of the boost or cut is in terms of affecting frequencies near the selected frequency.

The EQ is typically used to compensate for anomalies (peaks or dips) in the speakers response curve. However, many can't properly EQ by ear (myself included).

 

[Weigel21]

If you can, solder. I HATE trying to mess with anything that small when it comes to crimp connectors.

As said, the HPF is a High Pass Filter, which allows the frequencies above the said crossover point to be played unattenuated. Frequencies below the crossover point are attenuated with a roll off slope of xxDB/Octave. Your HU's HPF probably has a 12dB/Oct slope.

A bit odd that the HU's FLAT EQ setting boosts anything, usually a FLAT EQ setting is no boosts or cuts made with the EQ.

As said, a truly flat frequency response isn't desired (though drivers with a flat response are) funny as they may seem. Human ears are much more sensitive to frequencies within the typical vocal range (I think it's around 80Hz to upwards of 6kHz) and when frequencies get much higher or much lower than this range, our ears don't seem to find them as loud. Though most people's ears are more sensitive to higher frequencies than lower frequencies.

Now given most peoples ears aren't terribly sensitive to frequencies below roughly 80Hz, a lot would end up really boosting such a small driver to get the output to seem as loud as other frequencies. Which as said, will push the speaker beyond it's limits.