Thoughts on...SF Concertino Home (Existing) + SF Gravis I Sub (New)

I have an attic home office which is roughly 15’ x 20’ with a gabled ceiling. I have an existing pair of Sonus Faber Concertino Home bookshelf speakers that I am very happy with and I am considering supplementing with a pair of SF Gravis I subs to add a little oomph to the system. This is purely for 2-channel music listening. I would be feeding the subs from the same full range sends that go to the power amp for the bookshelfs and then adjusting via the crossover and level adjustment on the subs. 
Would love any thoughts, especially from anyone who has heard the Gravis I subs.