Passive speakers don't have high pass filters in them,
I wish they did sometimes, I had a pair of towers that were ported for around 42Hz and I played some EDM through them that hit the high 20's with the bass drops. In the battle of voice coil VS magnet--the magnet always wins.
It is hard to spec the crossover for the subs, that depends on several factors. If your speaker goes down to 40Hz before rolling off and you have a giant room node at 55Hz, that can be cured by crossing multiple subwoofers higher than 55Hz to fill in the room node by using mutliple subs located in different areas around your room. You can model your speakers if you have the T/S specs and calculate how much Xmax is used up at different power levels. Sometimes you can get the spec sheets from the manufacturer that include distortion specs at different power levels--select the crossover depending on how much power you use and what your limit is for distortion. Oh yeah, lots of luck getting that information unless you are using professional drivers! Never know and it won't hurt to ask.
I think what Russ is talking about is the acoustic roll off at the upper end of the woofer (or mid) if it has a smooth roll off of say 12dB/Oct at say 2KHz--using a 12dB/Oct crossover point at 2KHz would combine with the natural roll off to make it a much steeper 24dB/Oct roll off with acoustic/passive components. That works well, but does not apply on the low end.
Now if your speaker or subwoofer is active, most of them will have high pass filters in them to prevent damage and turning your woofers into expensive rice hats--very true.
For passive speakers, say you have some vintage Brand X speakers and a vintage Brand Y amp and you don't want to shoot the old cone across the room, you have two options. The easiest way is to use an electronic crossover and select a high pass filter to match your speakers to protect them from frequencies they can't produce. Some electronic crossovers have limiters built in so a limit can be set to protect the drivers from too much power either by cranking them too hard or accidental volume knob rotation. Plug that thing in between the preamp and the amp, make the settings and done.
The last option is to get a box from Eminence called the Defend. It "sees" the actual signal from the speaker wires from the output of your amp and will filter out the things your speaker can't do, limits the power level AND will protect the speaker in case your old school amp decides to dump the power supply to your speakers.
The Defend would be a great addition to some vintage speakers like the Altec Voice of the Theater to protect your investment. Blowing speaker drivers is never fun but if they are really old, sometimes you can't replace anything.
In theory, you could add a passive high pass to any speaker--the size of the caps and inductors will be HUGE but it could be done. It would be a waste of power, can create phase issues and various other things so the active filters generally are your best bet.