I've been using my model-I for a couple years now, I pretty much swear by it. I talked the band into buying 4 more double B1 systems, and another fiend who does a single picked one up.

I use double B1 but notice that synth lows are not handled by the double B1 system. I've tried additional bass systems out the bass out with and without the B1s, that solves the clipping and "bottoming-out". but as expected the make shift systems loose the Bose clarity. I am a little leary of adding tow more B1 and pPcklite, mainly due to the cost, plus I'd like to use equipment I already have.

When budget permits I may be interested in a SMALL Bose powered subwoofer - if Bose makes such an animal...but it'd be nice if it were designed for the PAS...

We play with a 5 piece band. We're using 4L1´s. 2 of them with a MM-bass in combination with 1 B1 parralel connected (for bassguitar and drums) . The combination MM-bass with B1 brings more bass and the B1 brings better attack.
Here you can see MM-bass:

http://mistermusic.eu/product.php?group=105

As you talk about "lows" with a synthesizer, depending on the 'voicing' (whether it is a relatively 'pure' sound vs. a complex sound with lots of overtones), you need to recognize that the B1's cut off fairly sharply below ~40Hz ... and you may be looking for sound below that.

For example, the last 1/2 octave of a full pipe-organ's pedal tones (E down to C) are noticeably weaker than the rest of the pedal notes. Likewise, on an 88-key keyboard with a "pure" (flute-like) tone the notes from the low E down to A are noticeably weaker than the rest of the range. FYI: This 'lack of below 40Hz' does not appreciably affect piano-like sounds because the complex dominant overtones of the piano sound are above 40Hz for even the lowest octave.)

Adding more B1's will NOT help those lowest tones (those below 40 Hz) -- although more B1's would help the next 2 octaves stay "in-balance" with the mid- & high-tones at louder volumes.

If you want those really low tones, you'll need to use an additional sub-system design to handle from 40Hz downward; I don't have any specific recommendations -- I'm still looking for a relatively portable, low-cost solution for that range, too!