You're generally better off getting it to peak at the resonant frequency of your car, which varies by car, but is usually in the 40's. Also, remember if you tune that high, you're going to lose almost all low end below 30-40hz, it's a trade off. I'd recommend tuning to around 40, and then cutting back the length slowly to increase tuning until you stop gaining output. It's a long process if you're trying to maximize SPL, but if you just want to have it be more towards spl while still being able to play music somewhat, try a tuning of around 36-40hz and see how you like that. All subs and all cars will react differently, so its all about experimenting and trying different things in your car to figure out what works for you.
50-60hz tuning is VERY high, and will most likely sound like absolute shit on music.