I have a pair of Sonus Faber Chameleon floorstanding loudspeakers which sound very good up top and in the middle. However, they are quite lacking in the lower registers and wonder if i should look at replacing them or adding a sub. I use Prima Luna Prologue Six monoblock power amps and know that they are not lacking in any way. The Sonus Faber's are sensitive too soo they sould have absolutely no bother driving them. Your thoughts please.