Employing filters and a powered subwoofer is much like bi-amping. Over the past few years I've bi-amped with multiple tube amps and SS amps. My Von Schweikert's bass module has a 4th-order crossover at 200 Hz. As I've mentioned above, I have a preamp which has a built-in filter which I can set at 40Hz to 160Hz. So I run it's output to my tube amps, with the filter set at 160, and then the tube amps to my mid/tweeter modules. I run the preamp's second set of outputs into my SS amp and then to the bass modules.
Then I measure volume levels, typically the SS output is a bit higher, so I insert a high quality passive "preamp" between the active preamp and SS amp, setting the volume level to whatever it needs to be in order to level the volume between the bass module & mid/tweeter module. I used test tones and a SPL meter to do this.
Have done this off & on for several years. Works very nicely with both my Inspire SE HO and a Korato KVA-20 European amp which has Audio Note output transformers and four 6CA7 output tubes to produce 20 wpc in PP triode mode. With the sub-160 signal filtered out, both of these modestly powered amps can play quite loudly.
Both tube amps sound fantastic when relieved of lower bass demands. And the integration between the bass and midrange is very good, as bass module & mid-tweeter modules were designed to work together as a single speaker. In fact the speaker's designer is an advocate for running them in this fashion.
Most speakers don't have a crossover this low. Bi-amping into, say a 3-way speaker with a crossover at 800Hz, would rob one of the wonderful lower midrange from a great amp like an Inspire SE. Also the mid/tweeter module has a nearly flat 8 ohm resistance curve, which is very friendly to a tube amp.
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