Yes, the 3/5 is VERY inefficient, and thankfully will fill the bottom somewhat with front wall boundary support. Yet I found them so anemic at 84 dB that I bought their big brothers the S3/1p and the great SC3 center channel (an S3 SE with TWO woofers, $700 ea) at 88dB to make a phenomenal, efficient front trio.
I do not know why Spendor doesn't market the SC3 sideways as a Dappolito pair. Sound EXACTLY like S3 SE on steroids.
Sorry to bend the thread, but I just find it more reasonable to use a 6.5" woofer or a second fiver to get +4dB efficiency rather than have to get a huge amp.
That said, I set up an S3 system for a friend with an NAD C541i and an Audio Refinement Complete Integrated with bookshelf (boundary support) mounting, and since he's ok without huge dynamics it works well. His wife poo-poohed the larger S3/1p. Sigh....