The MB-8 and PRO 60 are both rated at 150 rms. So anytime looking at an amp. You want to try to get as close as you can to the RMS ratings. And the ratings at 4 ohms which you will be running it at.
That rockford amp is rated at 60x4. The amp the sack shows is 120x4. Both at 4 ohms stereo.
That is the rating you need to look at. I like to run at least a 125x4 to the Double Up. We recommend the SYN-4 that is 125x4. Some are even running the SYN-6 to the Double Up, it will do 200x2 and 155x2 in 4 ch mode. (200 to the MB-8 and 155 to the PRO-60)
So the Rockford amp would be really under powered. Now since you have that amp. You could bridge it in 2 ch mode on the MB-8 and pick up another amp for the PRO 60's maybe. I cannot find the bridge power spec but it is probably 150x2 in 2 ch mode.
If you were running 2 pairs of 60ís or two pairs of 80ís. you can get a 2 channel amp like the SYN-2. But I prefer to run the Double Up with the MB-8 and 60ís on their own channels. So you can cross over the MB-8ís lower and get the most out of them.