Does your receiver have digital PEQ already? Many do. Also, does your receiver have room correction? That’s another way to get it to make corrections, even up in the speaker range. I would experiment with that first.
In this case, I would let it correct the whole curve, not just below Schroeder frequency. This is because the speaker badly needs correcting. The built in room eq might have enough horsepower to calculate all the corrections you need. I would re-measure at the end with REW to confirm you’re getting a nice smooth curve. That might be all you need. iIf your receiver’s DSP capability is too limited, it might not be able to make all of the corrections needed.
IMO, an “affordable“ amp+DSP combo that can also perform surround processing duties is the Monoprice HTP-1 with hypex amps like @Buckeye Amps. This will give you 16 PEQ slots of DSP per channel, plus state of the art Dirac room correction. And it can decode everything and has nice upmixers like Auro3D. It is not “the most affordable”—you can definitely remove features and pay less. But it’s very affordable for what you get.
For your multichannel situation, there is really no benefit with going fully active. You would need 2 amps per speaker, doubling amp costs, and more advanced hardware/software to make the digital crossovers. You could do this with a Trinnov unit, using 2 outputs per speaker and having it domthe crossovers, speaker eq, and room eq—but that is $$$!
What's the point of having the Klippel data if you're not going to use it at all and let the AVR just do the good old one fits all room correction?
I think the best way is to get around an issue like this is a MiniDSP 2*4 if your AVR have both pre-outs and pre-ins. it's cheap, and versatile.