Hard to say.. The "correct" advice here would be to start with better door speakers, seal and deaden the door, add an amp to the fronts, etc. If he still feels he needs more midbass after that, THEN other measures may need to be taken. Most subs can't play higher bass notes well, especially without shorting rings.. The z4 does use them, but it also is a pretty low effeciency driver so once you throw cabin gain on top of the low end, it simply doesn't give you enough output at 60-100hz, although it will play that region with flat response, just flat and reduced in output compared to the low end.
Anyways, I'm more than happy to spend gobs of money on subs that can play up to 250hz, just did in fact, to the tune of $750 lol.. Kinda hard to turn down a 21inch subwoofer that can also be used as a 21inch midbass though //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif High inductance subwoofers, frankly, sound like ****. Bass down to 20hz does basically nothing for me. You dont' hear 20hz well, at least not compared to most speakers mechanical self noises and the car itself making noise on such low notes.. Given that 25hz is where most bass starts sounding "bassy" and 50-60hz is where most of these retard thick coil, no shorting ring designs start to drop off, that's literally an octave of response.. IMO, that's not enough of the musical spectrum to be worth the money.. Especially when there are plently of woofers that can easily do 25-80 or even 25-100hz+.
There are two reasons why people cross their midbasses very low, neither of which is very intelligent, IME.
First. Most people are used to the sound of cheap speakers playing full range. They are used to their doors vibrating, feeling their pantlegs flat abit when the "bass" hits and all the intermodulation distortion from the lower tones bleeding into their midrange. So they attempt to recreate this sound with aftermarkets.. Since the laws of physics don't change that much from one 6.5 in a leaky door to another, that isnt' hard to do. Both aftermarket and stock speakers can easily be pushed into exhibititing all these negative characteristics that people often view as desireable..
Second. The SQ elitist crowd has been convinced it's the "Right way" to do things.. You have to run your midbasses down to 40hz or else your substage will draw to the rear.. This is because most of them are either competing in SQ competitions or are talking to those that do.. The problem is what works for a competition being judged and what works for daily aren't the same.. Just as a SPL fartbox might be tuned at 55hz for maximum SPL, its not good for daily.. Neither is most cars being judged for pure SQ. First off, most SQ guys aren't using the midbasses in the door, they are using kickpanels that are heavily braced, moreso than you'll get just about any door, period. This allows them to run them lower without rattling in the first place.. Secondly, most of these contests are judged at around 85db's with the car off. That's pretty queit and with no engine and road noise to overcome, making 6.5's playing 50hz tones a little more realistic, since they don't have to play them that loudly in the first place.. However, when your playing closer to 95db's with engine noise and road noise AND trying to overcome your 2 12's on 1600 watts going full bore, that little 6.5 is going to fail. (lets face it that's how 99.9% of you listen to music, so stop listening to SQ guys who are using their gear the exact opposite way)
Anyways the point where sound waves switch from being a pressure front to an actual sound wave is called the schroeder frequency. This is dependent on the size of the room the speaker is in. Below that frequency, in theory the soundwaves will be very hard to localize. Near this frequency, things become easier to localize and there will be nodes all over the room. Well above it and your dealing with point sources again. Generally speaking most small cars, like a hatchack won't go modal until well over 100hz.. I know in my large 2 door cadillac, my sub didnt' pull rear until 140hz, if everything was well deadened.. My current SUV isnt' modal until around 80hz and it's a pretty good sized vehicle. Unless your driving an escalade an 60hz steep sloped crossover just isn't necessary. Treat all your rattles and get a low distortion sub, if you havent' done those two things, why are you even worrying about SQ?
Anyways, sorry to go off on a tangent, but hopefully this info helped someone out and will cause them to try to some lower inductance subwoofers and higher crossover points on their mids.