Here's a professional and hobbiest experience with all this:
I recommend that you should strongly consider doing what THX found works, and that's to cross over your passive speakers system at 80hz, and let the active sub handle the much more demanding bass dubties - even with larger, more full range speaker systems. This free's up the receiver/amplifier to better handle the rest of the sound spectrum, relieving it of taxing bass dubties. This is especially helpful on receiver systems, with limited current deliver, and with passive speaker systems which offer limited efficiency and control going through passive crossover networks. Overall, you'll have much better efficiency, dynamic power, and range from the system this way.
This must be qualified however in direct respect to a "properly setup" and engineered sytem, fundamentally! Where people run into problems -and thus make all kinds of adjustments to overcome fundamental setup and acoustic issues - is that they most always never properly address issues such as critical speakers and seating, setup, and the acoustics in play, because they don't know what they're doing. As is too often the case, in directly adressing the main question being posted here, is that what happens is speakers will end up in acoustic holes in the bass response, making it impossible to get flat dynamic, accurate resonse from the system. Either the speakers, or the sub, or both will be sitting in a hole in the bass response, yielding a weak bass response. To try and compensate for this, what more often than not happens is that they'll move the crossover point to where things sound fuller. Or they'll crank up the bass/sub to try and overcome acoustic issues - and thus never properly adress the fundamental problem.
What needs to be done is to assure that the speakers are first flat (or at least not in a hole) at the critical crossover reigion. Ahd that goes for the sub too! Then, even with EQ assisting things, you can attain a solid, flat, accurate resonse from the system, with solid dyamic potential and range. You couple this with good bass managment in the above type of system, and you get simply tremendous performance results.
Basically, it all must be taken in context. Everything must balance out. If you ignore or aren't ware of all the issues in play, and don't properly adress things, you can easily make adjustments elsewhere in the system to try and mask over the real fundamental problems.
You can then see why results vary so extremely in regards to setting parameters and preferences from person to person, setup to setup, room to room, and equipment to equiopment. It's simply way to easy to not adress issues such as discussed here, as well as others such as toe-in, aim, speaker spacing, speaker and seating locations, speaker height, acoustics in the room, frequency response, water-fall plot, RT time, phase issues from speaker to speaker and/or seating location, speaker to boundary
interaction, etc, and so on.
Takes time to learn all of this, and it all adds up like ingredients in a master chef's recipie, or an experienced mechanic tweaking a race care.
If you don't know what does what, you're limited in results.
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"It's not that I'm so smart, it's just that I stay with problems longer." (AE)
"Sometimes one pays most for the things one gets for nothing." (A.E.)
"A person who never made a mistake never tried anything new" (AE))
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