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IMHO Mr. Karston is right about this: "But here’s something to keep in mind. If you don’t have an electronic crossover, this means that all frequencies are being fed to both amps"

I happen to use an electronic crossover made by Marchand Electronics; it allowed me to finally and completely resolve the problem of using a subwoofer with large 2-way speakers with no bloat or redundant amplification of certain frequencies. This unit is relatively compact, very transparent acoustically, and perfectly well behaved. There are other models that would readily allow you to cross over the signal at the required 550 Hz. You would have to choose the slopes for this (I used 24 dB/octave), but that is something the speaker manufacturer might comment on better than me.

It also would be better if both amps are relatively similar in gain. The electronic crossovers I’m familiar with (Marchand’s and others) have gain pots per channel, but the range is not nearly as wide as found in a preamp gain pot. These pots are typically +/- something lilke 6 dB total, and thus should be regarded more as "trim pots" than volume pots. All to say if both amps put out (for example) 26 dB gain, the crossover has to do less (if any) gain adjustment, in addition to division of frequencies fed to each amp.

Here is an overview of common active crossover applications in modern audio systems using miniDSP Device Console.

Yes active outboard crossover would technically be ideal but I really want to avoid going that route because it complicates things far too much. For example bypassimg my speakers complicated 3.5 way internal crossover and having the outboard active one sound as good? Tall order. Infact highly unlikely. Not a fan of DSP either unless it’s only in the low bass. I don’t really see a problem with feeding full range signal to a part of the loudspeaker even if low frequency are being blocked by the crossover and are turned into heat instead of drawing current from the amp to power the drivers, especially into high impedance load (above 10ohms) But there’s probably something I’m missing or are unaware of. With two gain matched amps there should be minimal differences that could be resolved with a passive analog line attenuator, like one mentioned above. No?