e39 sedan sound quality

Under the driver's seat is a common position for a *deep* sub if you want to feel the rumble (and I mean feel it).

But soft clear bass doesn't exist - low frequencies are non-directional so you tend to hear them at (roughly) the same volume throughout the listening space. You only need a single sub unless you're making a high-end Dolby or DTS system that has been mixed with two separate sub-woofer signals during production

Clarity comes from the high frequencies which are increasingly directional with increasing frequency. 

Thanks for a reply, there is a thing about under the seat. I want it but its not possible in my car its too narrow and have air wents. So the option on rear deck. So 2 subs makes no sense?  Im thinking about hertz energy es 250 is it good ? Photo is from internet of my e39. And i have holes here just need to do it wider and its thin metal here. If i decided to make it 1 sub i should place it on ski hole. There is any difference sound from ski hole or rear deck bouncing from window? Sorry for so many questions just don't want to do unnecessary mistakes

You mean a crossover with a high-pass at 250Hz?

Down there there's nowhere near enough directional information to justify two subwoofers unless you just want more OOMPH! Which is perfectly reasonable.

No, i mean Hertz HX 250 D, RMS 450W the brand "Hertz or Hertz Energy ES 250.5, 250 RMS

1 person told me do it without a box you will have less noise of metals around. Can't do good a box in a box(trunk) Because e39 trunk is a metal can...
And i love and hate this car at the same time... i just want the music to sound perfectly but it will not i suppose..
And thinking if i did good at the front i set there focal 165 as they are baseless mid speakers because i did it with a adapter to set a bigger diameter speaker here from 5" to 6.5 and it doesn't have a sealed box

Thanks !

Photos are just example from the internet

Sorry for the slow reply, I missed this somehow... I do that.

When I had a car with a parcel shelf that's how I did it - but I used two wide-range drivers.

One thing to be aware of is that the pressure wave in the car will dampen your music beyond a certain pressure level (volume). The reflections and all sorts of other effects come into play too - getting audio to play *well* in a car is an art. Getting it to play well at volume is an art and a science.

Now I didn't tell you this ... (cough, cough) but if you want to improve the bass response you should make sure the back of the woofer(s) are open to the outside air. I suppose it almost goes without saying that this is difficult (with rain and such) but if you can port the "rear" pressure wave to the outside air, people around you will hate your guts but you will get MUCH deeper and louder bass response.