All crossovers are "in line" with the signals. Can you tell us more about what the context is and what you are trying to achieve?
A plug-in high-pass filter like you describe will roll off the bottom end of your main speakers at 6 dB per octave (it's a capacitor selected with the amplifier's input impedance and desired rolloff point in mind).
Whether or not it will help get a better blend with your subwoofers, I can't predict from here.
Are you already using some sort of high-pass filter on your main speakers?
Are your main speakers sealed or vented (makes a difference in how rapid their inherent rolloff is)?
How steep is the low-pass filter on your subwoofer?
What ballpark crossover frequency are you using?
Thanks for taking the time to respond to my post. Guess I opened a can of worms. I'll try to answer your questions best I can. This experiment came about when I installed a pair of in-line filters I had on hand which xover at 65hz. After a few minutes ajdusting the low pass on my subs (which only go down to 40hz), I was relatively pleased by the results (which cleaned up the bass quite a bit) and now assume I can do even better. My main speakers are front vented (VSA 4JR) and I run a pair of old Velodyne HGS-10 subs crossed over at 40hz (I do not know how steep the low pass filter is on the subs-are you referring to slope?). The NHT X2 filter Bob-reynolds (thank you) suggested is intriquing as well.
Thanks for replying.
Okay, when doing a crossover I try to get a symmetrical slope on both sides of the crossover frequency. In my opinion, this applies to subwoofers as well.
The Velodyne HGS-10 has a second-order (12 dB per octave) variable low-pass filter. So, we'd like for your main speakers to also have a second order rolloff (12 dB per octave).
Your main speakers, being vented, have a 24 dB per octave (fourth order) rolloff below the port tuning frequency - this is inherent in the breed. If I understand correctly, your speakers are the Von Schweikert VR-4 Jr, with claimed bass extension to 23 Hz (-2 dB). Is that correct? If so... well, this gets a bit complicated because your main speakers' inherent bass rolloff is way too low to be part of the crossover to your subs, but on the other hand I think that can be changed.
I suggest stuffing the port tightly with closed-cell foam which will turn the box into a low-Q quasi-sealed enclosure. The bass will then probably (or should I say hopefully) roll off at 6 dB per octave starting somewhere above 40 Hz, transitioning to 12 dB per octave somewhere well below 40 Hz. I do not know the exact frequency that would work best for your in-line filter, but your 65 Hz filter might work just fine. [Make sure that your in-line filter is designed for the specific input impedance of your amplifier, as the frequency where it starts rolling off will be different otherwise.]
The idea is to change your bass enclosure's characteristics to give an inherent 6 dB per octave acoustic rolloff in the crossover region, and to accelerate that by using your in-line filter so that you get a net 12 dB per octave high-pass characteristic. You should be able to match this up well with your subs given their variable-frequency 12 dB per octave low-pass filter.
Just to see if this idea is in the ballpark, try the closed-cell foam with your present filter before investing in a new in-line filter. I'm guessing that you'd use a setting of around 50 to 60 Hz for the sub's low-pass filter. I would also guess that a 50 Hz in-line high-pass filter would probably be closer to ideal (along with a lower low-pass setting on your sub), but try what you have on hand first.
No guarantee that this would work of course, but it's what I'd attempt.
The NHT X2 is really tough to beat. It has symmetrical filters as Duke suggests (2nd order), support for balanced and unbalanced lines and 3 high-pass settings.
A good friend just added one to his Lavry DA10, Pass Labs X1, Parasound JC-1 amps (actually bought from Duke), ML SL-3 and ML Descent system. This allowed him to run balanced lines from the source to the amps and he regrets not doing this a long long time ago.
Thanks again for the time you devoted to my question. I'll try your suggestions and report back. Certainly the possibilities of what could be (thank you bob_reynolds) have me thinking about getting serious about implementing a more sophisticated ("purpose-designed" per Kr4) crossover like the NHT X2. Any suggestions?