a ZX500.1 will work fine on those L5s, when I had my ZX300.1 on it, you could tell they could handle much more, plus I had them in 2 seperate truck boxes cuz I never had time to build another one so they could've been a lot louder just because of the box. But if he doesn't want to go with those, there are tons of good priced 10s and 12s that are decent that'll get loud
BTW: 1 10" L5 could handle 450w RMS, so that as long as you're running that ZX500.1 on both, that'll work fine