Hmm... Well if you do want a bit more of a pummelling, try to move the left-hand band upwards. I've found I can feel them when I do that, although it's too much for me. You also have to be careful how high you go because they start to make a 'popping' sound (a bit like someone shaking a sheet of metal, Rolf Harris style).
I'm over the moon with mine - my only real complaint is that you do have to turn it up to get the nice highs and lows... At lower speeds you don't seem to hear them much, which is odd given that the Carver technology is supposed to increase bass at low volumes lol! I guess you could say that the subs could provide more 'thump' and I think the reason why they don't is because they're actually quite small as subs go (although the mesh over them looks large, the actual speaker diameter is pretty tiny for a sub - it's because behind the mesh is a cage design that's noticably smaller than the mesh, then there's the panel with the speaker attached that sits on top of the cage which has to take into account a lip around the cage which is what it secures by). The rule of thumb is that a small speaker can be more 'punchy' and accurate simply because there's less of it, and air, to move. The downside is that the amount of air moved relates to how much of that really low-down grumble a speaker can produce - it's obvious really, small and punchy is going to mean a quick and short sound whereas large and slow is going to mean a slower, longer-lasting grumble. I'd guess that the point at which our subs start going all 'poppy' is when larger subs would really start to make loud rumblings.
To fit bigger subs that would produce more of the low-down rumble you're looking for, i'd expect it to be a fairly big job: I don't think there'd be much problem with connecting such things up as I assume the amp uses normal impendences etc (unlike Bose systems - if you want new speakers, you have to replace the amp and ideally all the wiring too on them), but you would have to take out the entire plastic area behind the seats to reveal the cages onto which the current subs + plates fit. You'd then want to either cut big chunks out of them or replace them with new cages so that you can make new plates with your new subs mounted...
As far as i'm aware, there's 3 speaker configs in the Z4: the standard 6 speaker setup which consists of 6 speakers (suprise!) that're powered directly from the stereo headunit, the 10 speaker system that doesn't have any additional adjustment over the 6 speaker system (just the usual bass/treble/bal controls) but (I think) has an external amp (maybe the DSP amp, maybe a different one). Finally, there's the DSP system which is the one with the 7-band EQ, 10 speakers and the external amp. I can tell you that the tweeters and subs in this system are manufactured by Philips, so I'd guess that the rest is (possibly also including the amp). From memory, Philips also make the stereo headunit. This system is the THX-approved system. Carver is a technology name that's supposed to boost bass at low volumes and also relates to the claim about how good the subs are despite their tiny size (something, from memory, to do with the subs having more forwards/backwards movement than usual in an attempt to negate their lack of diameter).
Hope that helps!
Bose stuff is generally pretty horrid, so how poor the standard system must be, I don't know!