best used floorstanding speakers under 5k that has small footprint

It’s hard to find speakers with mids similar to the ATCs except by going up the line.

What improvements are you hoping to get over the SCM40s? More bass for the same footprint? Probably unlikely without a bigger cabinet.

Things I’d look into are adding a good sub (or 2), upgrading the SCM40s to active, upgrading them to the v2 new tweeter (I’d email ATC to ask if both these are possible to retrofit from the v1s)

Also, what amp and sources are you using currently? Biamping or triamping might be another idea.

Just scouring the current listings of floorstanders in your budget on Audiogon right now and nothing jumped out to me as a real upgrade.

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.