@gdnrbob, yes Richard Vandersteen's rather unique practice of using a woofer below it's natural resonance frequency works extremely well. I have not seen mention of anyone else using this approach.
High passing the main speakers makes so much sense I'm surprised it is not more widely adopted. As you say: it unloads the amp essentially providing more headroom and reduces speaker IM distortion.
I have a DIY line stage where I can select between output terminals and have one of them with a single cap and resistor soldered in place forming a simple line level high-pass. For those interested who do not want to butcher their kit just connect it to either end of your interconnect cables.
The value of cap depends on the input impedance of the power amp or the internal impedance of the tape loop in your integrated amp.
Google PLLXO (passive line level cross over) for tables and simple formula. This is not only the least expensive way to do this but also probably causes the least damage by avoiding extra cables and their associated connectors.