No, a subwoofer crossover does not usually include a high-pass filter (a filter that blocks the very low notes from the satellites). That's what makes it a subwoofer and not a woofer.
The way you set up the sub xover is to try to match it with the natural rolloff of the floorstanders, the curve of which is shaped like a high-pass crossover anyway. You should try to set the crossiver frequency on your amp to the F3 of the speakers - in other words, the frequency at which the signal is 3 db quieter than the frequencies above it. So if your speaker is flat until 65 Hz, its F3 might be around 55-60 hz. So turn the dial to there and then go by ear.
The problem, of course, is that you don't know what the F3 of your speakers is and you probably can't believe what the manufacturer says. For example, ProAc claims that the Studio 100 has a frequency range down to 35 Hz and a sensitivity of 88 db at 1 watt/meter. OK, these are classic speakers and I really like them, but come on...from a 13 litre speaker? It just is not possible. But of course they don't say how far down the speaker is at 35 Hz. Maybe it's -10 db in which case it might be true. But you would need to know the point at which anything below that frequency is down more than 3 db.
An additional problem tuning these things is that an average-sized room is going to give you some gain below 40 hz (thus exaggerating the low bass) and will probably suck out a good -6 db between 70-100 Hz - right where the sub crossover is. So even if you go get a sound meter and test tones and try to figure this all out, you still are going to have to fiddle with the crossover frequency and level to get it to sound right once it's in the listening position. So I will save you a lot of time and tell you to start around 50 Hz for floorstanders and around 70 Hz for standmounts and sart tweaking from there.
But one major point about your post: A subwoofer will NOT ALLOW YOU TO PLAY LOUDER. It will give you more (and hopefully better) bass at the volumes your speakers already play comfortably. But they will not let you play louder. For that you need, as you said, a crossover between your sub and your midrange - 3-way speakers in other words. Then your sensitivity could improve and you could go louder.
The only thing you might try - and this depends on your speakers a lot - but if they are ported speakers, you could try stuffing the ports. This will negatively impact the bass from your floorstanders - you are effectively turning them into sealed speakers. But you can compensate with a higher sub crossover frequency, and it might give you 1-2 db of extra sensitivity - in other words, you could go a little louder and combined with the extra bass from the sub, could really help a lot. It might also make your standmounts sound worse but it's worth trying - just stuff the ports with socks.
Best of luck!
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