Stick with the Celestions until you get out of that room - problem solved...
The Gallos IIRC have side firing bass drivers. I can't recall if the pairs are mirror imaged. If they are you might want to see if you can swap them around.
This way you might help prevent side wall reflections from the bass driver.
If you have to place the speakers too near the side walls, try to use some good absorbers on the sides and backs to minimise reflections, and again at the first reflection point.
The Gallo tweeter has IIRC 300+ degrees of dispersion. It's possible (likely?) that the system has been voiced for reflected energy that would be expected from nearby walls. You don't have those nearby walls,, so the lack of such energy might make the speaker go dull. The Celestion, OTOH, has a tradional tweeter that provides the bulk of its energy on axis. That might explain a lot, but it's just a guess.
BTW, if the Celestion subwoofer was placed properly, it will provide smooth bottom end response while the Gallo woofers will have unpredictable peaks and valleys since the speakers were placed far from the walls. You might be getting some combination of bass destruction/reinforcement that exacerbates the sense of diminished high end response. Again, just a guess.
I think the most actionable guidance is to stick with the Celestions until I can get out of the basement. Death of spouse or marriage may be critical predecessor event though, or I convince spouse to move so she can have a decent home office (weak excuse) and I can have a stereo room (the real reason). I'm so torqued up over this I'm considering an equalizer. I had one in the late '70s and it was magic on an Advent speaker, but god knows now.
This may sound odd, but as another option to try, I've actually heard of the Gallo's being used in a corner placement. This might even allow you to get away with a bit closer wall location. Be sure to diffuse the first wall reflection as mentioned above.
Above all, sure hope that coming back to the problem on a better day, maybe aided with your favorite beverage and some great tunes, will restore the Fun Factor to your experimenting.
Sometimes a shitty room is just a shitty room, no fixes possible. However, one thing that has not been mentioned is the use of something like Trinnov or Dirac room correction.
The Trinnov can be had by purchasing a Sherbourn 972 receiver for around $500. Dirac sells a standalone unit for about $20k - OUCH! However, Emotiva is supposedly soon releasing the XMC-1 preprocessor with a Dirac implementation. This will cost you $2k.
Finally, try facing the woofers in on you Gallos and then experiment with toeing in the speakers. Start extreme and work your way out, see if that helps...
PS: I have Ref. 3.1s and Ref. AVs, hence my suggestions...
Well, I found the fix - Focal Arai 926 speakers. I was kicking myself as I bought them because they are HUGE for he room and I just knew they'd sound horrible but I couldn't help myself. Actually, they trounced the Gallos wholeheartedly, and the Celestions, which sounded so sweet at lower listening levels, became screeching harpies at higher listening levels (Montrose demands a certain SPL) whereas the Focals just pumped out clean sound that didn't make me wince above 5KHz. I'm done.