What does an impedance-matching issue "sound" like?

For BiAmping, you really should have crossover, otherwise the amps see an infinite impedance out of band.

A simple 6db HP crossover can be effected with a single capacitor for the upper and a LP for the low with a choke.

It would be better to use slopes similar to the in-built crossover.

For a quick cheap & cheerful XO, pick up a dbx 223 or similar. It's 4th order or 24db/octave. 

@almarg - thanks, I have now experimented with the vertical bi-amping. It’s probably too early to have a firm opinion but as you noted, I noticed a little weaker bass, if not a smoother mid/hi top end. Also strangely enough it sounds like the stereo field is a little smaller than when I was mono strapping. Funny thing, the song I noticed the "breakup" from my OP was the arpeggiated rhythm guitar part in Tears for Fears’ "Woman in Chains," which now in the bi-amp configuration is NOT mostly placed in the left stereo sound stage but is now spread out across the entire sound field. Very weird as I didn’t change the output config of the preamp at all.

@ieales - the Rival has an external passive 3 way crossover. I’m sending one channel now into the mid-high input and the other channel into the bass input. I’m not sure how I would implement an active crossover into that configuration as the external crossover is built specifically for the Rival and from what I understand must be used (you cannot connect a direct signal on the speaker inputs).

Here is a photo of the bi-amp connection into the Rival passive crossover

https://jsqt.smugmug.com/Luxman-MQ-88uSE-Diagrams-Measurements/n-cgz5rM/i-dFFnJ5v/A

It's a shame they don't give the poweramp (blue) distortion figures when it's in "strapped paralleled" mode also, this "could" give your problem.

Cheers George

Based on about 5 hours of listening tonight in vertical biamp config, I’ve gone back to running the amps in parallel.

The bi-amping sounded a little smoother I think in the higher frequency. But there was an odd effect on the stereo image, seemed more like 2 separate speakers than a “disappearing” stereo field. 

At first I just couldn’t figure out what was causing the effect on the stereo image. Voices seemed to be part of the mix rather than having their own center showcase. And other elements of the sound, especially the bass, seemed more a part of the stereo field than having their own pinpoint placement in the mix.

Overall it just seemed like a “stereo” versus a staging of a live in the room placement.

i finally figured that the issue was that since the bass was compromised by this configuration, the deficiencies in bass caused the sound to be less anchored overall in the stereo field.

Anyway, that’s my best guess.

As I mentioned before, reconnecting everything seemed to correct most of the initial OP issues I had with the mono strap configuration and now that I have gone back to that config, the stereo field is back to normal. Voices are better anchored in the center of the mix. Bass is tighter and more focused. Stereo field extends beyond the speakers again, and the speakers have disappeared more for better imaging.

So it’s been quite an adventure. In many ways I am back to where I started but the Fidelity has been restored and damned if I don’t know a whole lot more about this whole process than I did in my original post.

Thanks to all who helped sort this out