The life expectancy on the subs, when ran at RMS, should be about, forever. As long as gains are set properly and nothing is being clipped. The Skar IVX, SSA Dcon, and Sundown E-series are all great options for good quality on a budget. There's also a Dayton HO, which are known to need an even smaller box. The nice thing with the Dcons is the higher sensitivity.Nice to see you getting away from the Kenwoods, cus those are garbage. For a good indicator of lifespan on these other subs, I've been running my SA-10 on double or more the RMS for almost a year now and haven't had a problem.
Just for reference, what amp are you using?