New Smart there, now the sound is missing! (Question about Boxes/Subwoofer)

Hi has been a member of the club for two days, but as a second car I got a 2002 Smart cdi pure convertible. I’m happy with the car at the latest since refueling (3.5l without sneaking), but what’s not possible is the sound (the one from the boxes). Don’t get wrong, from the age where it has to be done so loudly “BUMM! BUMM!”, that you can hear it next to a starting tornado, I’m out. In my 2002 Smart afaik the standard 10er boxes without bass reflex housing are installed, because the in 2000 These cardboard things are of course the last scrap, because the hands-free device of my mobile phone sounds better. So I want to soon get to the SC to have the things retrofitted (cost the pair about 50€). Because they are not only good for the sound, but also ensure that 13.0cm speakers fit in (otherwise only 10.0). Meaningful Pionieer TS A1303. In addition a matching CD/MP3 radio of the “100€ price class” with corresponding performance. I have noc h don’t think about it. Aja, now the most important thing. Since 13 boxes produce hardly/no “pressure”, I thought to install an active subwoofer under one of the seats. Does anyone know how high the maximum installation depth can be? Ideally someone knows a suitable system up to 100€?! Thought about it. Do you think that fits under it? I don’t want to build a small trunk with a bandpass tube! Or do I still have a thinking error somewhere else or do you have a better idea?! For example, good 13s that make a subwoofer unnecessary? As I said, it’s not about volume, but just about being able to hear “ordinary” music. Thank you for your ideas!