Sub distortion and amp power

[832ark]

Hi, hoping someone can help with this. I've just fitted an 8" Alpine sub to a ski hatch enclosure in a 3 series. Previously I was running an amplified box in the boot as a temporary measure without issue. The problem I'm having is quite a lot of distortion at very low volume levels. I have a suspicion but I'm looking for other opinions - I'm wondering if the amp is not sufficiently powerful? I thought I'd try an Alpine amp that I just happened to have in the garage which is rated at 140w RMS, the sub is rated at 350w RMS - does the amp just not have the balls to run this sub?

Many thanks!