But I was just worried because an SCM-40 is a substantial floorstander to look at my room normally works best with big bookshelves rather than floorstanders
ATC are criticically damped in the bass. You don't get big bass with ATC - wrong type speaker if you want strong heavy bass (Wilson or B&W for example are much heavier in the bass department). Unlike most floorstanders they will not overpower a room with bass (if anything, even though they are very tight, dynamic and punchy, most people tend to find them too shy or understated in the bass - this can be an issue for soft listening levels where you could need some tone control to boost the sound)