This is incorrect.
Base Stereo has tweeter grilles, but shine a light into them and you can see there's no tweeter back there.
You shouldn't call any underseat speakers "subwoofers" in any BMW, but the base Stereo cars it's really a misnomer.
There are 6 speakers in a base Stereo car, four 4-inch and two 6-inch. They look like this:
The signal coming out of the iDrive when it is coded for base Stereo mode is VERY odd.
The front signal is heavily boosted in the bass, and less so in the highs, to make up a bit for the missing tweeters and the missing 8" and the missing amp. The front channels drive the underseat speaker and the front door speaker. This is like a 9dB boost:
The rear output has the low bass filtered and the midbass boosted:
We do have plug-and-play upgrades for the E70 with Stereo, if you're interested contact us. We've helped out a number of people in that situation.
I don't think there is a tweeter on any of the centers, either, 676 HiFi or 677 Top.