I've previously used a parametric equaliser (PEQ), tuned differently for left and right channels, to address the worst of it but have now come across a more powerful method that I thought I'd share here: Room correction via stereo impulse response convolution.

Here's the general outline how this is done:
1. capture your left and right speaker response curves in REW as usual (I'll omit detail here and there are high quality guides on this)
2. Per speaker, Use REW's EQ to automatically fit the measurement to a target curve. Click "Save filter as new measurement" for each EQ.
3. Select File -> Export -> Export Impulse Response as WAV, choosing your two new measurements for the left and right channel respectively. Choose a 32-bit float stereo, Export min-phase version of IR and place t=0 at sample index: (default value).
4. Load this wav file into your IR convolution reverb plugin of choice (or Pianoteq reverb directly). Make sure it's the last step before output.

This results in a significantly cleaner room correction than a calibrated PEQ, and is capable of addressing pretty egregious spikes without tonal changes or incoherence. It brings the sound very close to headphone level clarity, but still connects the sound to the ambience of your physical space - and it let's your woofer do its bodywork.

I'm pretty thrilled with this discovery, it significantly improves the audio quality of any output through these speakers. For Pianoteq in particular, incorporating the piano into my room's acoustics is really quite transformative, solving the "in the void" problem that you can have with headphones, without having to accept muddiness/boxiness. I recommend it!