Since these speakers are about $450 each, no matter how good they sound I would put very little money into them. Modifications will not increase their resale value when you get the urge to upgrade, but mods are fun. I have modded my last 3 speakers with positive results.
The UF5's are built to a price point, you could easily spend $500 on mods but then you would have $1400 speakers with a resale value of $500. What amp/receiver are you using? The $500 might be better spent toward upgrading your electronics and make a bigger overall improvement than changing the crossover.
DIY mods would give you the most bang for the buck. The UF5's weigh only 40 lbs each, resonance control would be a good place to start. Benefits would include a tighter tuneful bass, greater midrange clarity with a tighter focus and improved 3-D imaging because cabinet vibrations are reduced.
I have documented many of my cheap tweaks that can transform an affordable speaker into a giant killer here:
https://www.audiocircle.com/index.php?topic=44748.msg400398#msg400398Cabinet bracing along with damping both the box and the woofer drivers will go a long way toward improving the sound. Tap the speakers sides, do they sound hollow or a well damped thunk? If hollow then cabinet modifications should be #1 on your list.
No Rez damping material has been mentioned a few times. Great stuff but 2 layers of self stick vinyl floor tile from Home Depot, 1" open cell foam, and 2 cans of 3M Hi-Strength 90 spray adhesive (for both the tile and foam) works almost as well, $200 vs. $65, your choice.
Damping the woofer baskets with PlastiClay will add clarity, is cheap and fun to do. Frank Van Alstine first wrote about this tweak in the very first
Audio Basics newsletter, Volume One Number One, January 1982, which is still available on the AVA website, and it is still as valid today as it was 38 years ago. The results were just as Frank described with clearer, less colored, quicker, and smoother sound.
