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. 

I have heard a home theatre system with the Sonettos and Gravis subs and it was exception given the price point.  I would be surprised if the subs didn't provide you what you are looking for.

I personally would be more likely to upgrade the amp section than add a sub based on my bias that a 2 channel system isn't perfect and that's part of the experience.  And some of my friends consider that approach as wrong as a football bat.

Let us know your decision...