A gallo reference sub amp or second subwoofer?

Hey guys, I have a set of gallo ref 3.5 that I have had for about 4-5 months. I love em and combined with my revel b120 sub they go plenty deep. However I have a chance to buy a nib gallo ref sub amp for less than 600.00. Initially I was going to buy a second revel sub for stereo, but the gallo is about the same cost. I currently have an audible illusions modulus pre and emotiva xpa2 amp. Everything sounds fine, but they can always be better. My future includes an emotive sira line stage and either rogue m150s or 180s, possibly a rogue Zeus. I therefore may not always have 300w of ss to manage the bottom octaves. With that in mind which route would everyone recommend?