Alembic_hawaii, just to answer one of your question, having the PU in different position DOES make a BIG difference to the sound, period.
Then, I personally like single middle PU too, and maybe why you are so happy with your instruments, is because it is positioned at a nice spot.
Saying that, I like the sound of the bridge PU mixed with the neck too...
Personally, I like to take advantage of options when they are available, and everyone think different, so do I.
Bootsy Collins for instance, is well know for his eccentricity, and I like him, his sound and his style.
You know, there are basses with a single PU close to the bridge, some close to the neck, some in the middle, and so on...
Yet, do they all sound different? Absolutely.
Every-night, I plug my basses and jam other various songs, and I keep swapping, none of my bass gets ignored, yet, I can tell you one thing for sure, they all sound different and some work best with one song, and some work best with other - NO MATTER what setting you use or fingering style or strings... It is nice to have tonal versatility.
One the other hand, if you play with a band, and are representing the bottom end, and this bottom end is the signature of your band, then in that respect, and I can understand why one, would stick witch the same instruments/sound.
And let's NOT forget, Dave Houk has got a three PUs Alembic, and if I remember right, he said that he preferred the mid PU over the neck PU.
http://club.alembic.com/Images/393/31801.html?1159926964