How are Proac 1 SC's at low volume levels?

Use acoustic room treatment if you want to hear more at low SPLs. Speaker selection is key of course, but if your room is tweaked acoustically it will really help with low level listening IMO.

Exceptional midrange is the most crucial factor in low volume listening and the 1sc's are indeed exceptional in that regard. Even better---if you can find a pair---are the ProAc Tablette 50 Signatures. The Sigs are noticeably more efficient as well. Good listening!

I have a pair. A small room, 11x12. I drive them with 60 watt tube mono blocks. As for low levels, don't really know what your 'low' is. Typically I play mine at 75-80 db. Great sound. I also have a pair of ProAc 140's and they also are tubed. My room has been extensively treated so it is on the 'dead' size. Mostly play small combo jazz, vocal and instrumental. In the end the only way to tell is to buy a pair and put them in your chain. Since the 1sc's are always in demand, you can always sell them off at little or no loss.