best used floorstanding speakers under 5k that has small footprint

What size footprint are we talking here? Women's size 8?

A good amp is a must for all ATCs. Upgrading your power amp or biamping might be a real option if that’s one of your issues. The ATC P1 and P2 power amps (and the SIA2-150 integrated) are great matches to the passive speakers, and come with the link feature to enable easy biamping/triamping, though this can also be done with any suitable power amps running the same gain with some splitters off your preamp outputs.

Then again, for the price of an ATC passive + amp, one could usually have gotten the active speaker version, which should be superior.

If it’s possible to get a loan/trial, I’d experiment by seeing what improvements a single high quality power amp can bring, or even 2 x identical high quality power amps would do biamping; driving the woofers of each speaker from one amp, and then the tweeter and mid from the other amp.

Check out Spendors. The A-line and D-line have a small footprint. I have owned Spendors and they are an outstanding floorstanding speaker!

Vandy Treo's!
Bob

V2 of your speakers, spendor, vandy treo, joseph audio...