Holy Crap What have I done?

Some different thoughts here: you said when you added the crossover to the system, it sucked all the air out of the music. Are you talking about the portrayal of space, harmonic richness, textures, etc.? If so, the crossover betwen your preamp and amp could ultimately do far more harm than good.

Time and time again, I have found the most critical cable in the system to be the preamp-to-amp link. I have achieved incredible musical magic with the sonic attributes described above. But I can so easily destroy all of this with a change in the IC in this link. And with a crossover here, you have two such ICs in the critical mids/highs.....not to mention all these extra active circuit stages driven by a rather wimpy power supply to boot.

Changing from 100hz to 85hz is just not gonna do anything if you have problems in the upper frequencies. Try the crossover out of the main link and have it drive only the subwoofer as a low-pass and let the main speakers run full range. You lose the benefit of taking the bottom octaves out of the main amp and thus bringing on more clarity in the mids/highs.....but you get the crossover and extra IC out of the loop and this alone can be a huge benefit.

I have found crossovers and equalizers to be fine up to a certain level of system refinement. And yes, they can be beneficial to compensate for low-frequency room nodes, but they can so quickly wipe out the dimensionality and dynamic contrasts that you may have worked so hard to achieve up to that point.

John

Set Lowpass Filters A & B to 250Hz.
You want the filter frequencies to be as far apart as not to interfere with the filter that is upstream (first) in the chain that is actually doing the work.

You have 2 different choices for the input

Left & Right Line Level Inputs: Use either or both of these inputs. You will need to set the Input Level and Lowpass Filters A & B. The (input level) volume needs to be set at the same volume as your main speakers to get a seamless blend.
When using the Theater input, the volume on the subwoofer is bypassed and will be controlled by your processor/receiver and maybe your crossover level controls.

Theater Input: Connect to this input from your sub output on your receiver, preamp or processor. Set Lowpass Filters A & B to 250Hz. Input Level is bypassed and volume is controlled from your system's subwoofer volume control or maybe the crossover level controls.

Hope this helps.

Man is this confusing! How can you put a crossover between the amp and preamp? Is this only in the subwoofer circuit? Seems to me that the inline filter would affect both the sub and the monitors, which would be the effect of more bass slam at the expense of mids and highs. This is what I think, and you guys tell me if I'm wrong. What Aggielaw originally had was a pair of speakers with a two-way internal passive crossover inside the box, and an ACI sub with a low-pass passive filter inside the box. If he adds a crossover in front of those crossovers, especially if it is a two way, he will really mess up the passive crossovers in the speakers. Unless I'm just not getting it, I would believe this setup is not good. And again, just my opinion, but I see no problem with the monitors rolling off naturally at 65Hz. I guess I don't agree with that article you read, Aggielaw.

"How can you put a crossover between the amp and preamp?"

It's called an active crossover. Read this for further details.

Besides the Marchand products, Bryston makes the very nice 10B that comes in a version optimal for subwoofers. If you really want to go high-end, the Pass XVR1 is quite a statement product.

The last thing I would ever want to do is have a subwoofer (that is optimal in the 85hz and below range) handle the very critical upper bass/lower midrange of the 3rd and 4th octaves. You clearly do NOT want the sub AND the mains both running in the 65-250 hz region! That will be a disaster to the instruments in this range. Keep the lowpass as low as you can go, just a tad above the natural roll-off of the mains and let the mains run full signal. You will certainly get the bass extension you want. The only issues will be coherency between the sub and the mains and finding an optimal location for the sub.

Please let us know how things progress.

John

Jafox...I beg to differ with you about running main speakers full range and letting their natural bass rolloff occur.
Of course the obvious reason not to do this is to keep the LF signal out of the main power amp. But it's good to keep the LF out of the speaker as well. Although the acoustic output may roll off, the woofer will still be trying, ineffectually, to reproduce those subwoofer frequencies. It doesn't help the upper bass and midrange to have the cone floping around unnecessarily.