I don't know what you normally bowl on, but on true long or heavy sport patterns, the 919C won't match the Nano on performance. The cover is stronger than the 919C, especially once it is sanded down for the heavy stuff.
I'm not a big fan of sticking with one manufacturer or family. I found that out the hard way when I switched to all Lane 1 only to realize that the only 2 Lane 1 balls I liked were the two that convinced me to switch. I bought 4 used balls and have since sold 3 of them. The 4th is only around because I haven't been able to sell it yet. I was really looking forward to that arsenal too; my Cobalt Bomb Pearl is the best ball I've ever had, period. Anyway, my point is that liking one ball doesn't mean you will like ever ball.
If you really want to stick with the Ebonite family, my choice would be:
Heavy: Ebonite Mission X
Med-Heavy: Hammer Taboo/Taboo Pearl
Benchmark: Vital Energy
Med/Light: Track 503C
Light: Track 300C/Blue Hammer (if you like Urethane)
I built this on the assumption that you want smoother equipment. The new Track 811A SE looks promising, but it appears to be very snappy. The original Taboo is a better choice for this, but the Blue/Silver comes with the free spare ball and would add some diversity and length to the top of your line-up.