Tonight I watched a duo acoustic act with I think a Bose L1 PA positioned behind them (great as a monitor then) with no feedback! Just a small maybe 4 channel mixer for guitar and 2 vocal mics. OK is the no-feedback considering the placement, a factor of the Bose having such a skinny profile. I would like to pull that off with my AltoTrooper pa ..any hints .I assume just angle it off axis enough from my mic?
 
 
 
 
 
 
 

A band I was in used 2 of those and a total of 4 sub-woofers for our entire PA.  We had ZERO need for monitors.  Unbelievably good sound dispersion and even with 4 and 5 part vocals, no feedback.
 
LOVED those things - BRILLIANT.
 
Have your music store set up a demo and you can prance around with a mic to see how it fares for yourself.
 
Bob Bone
 

Yes, those things are amazing. My duo regularly uses an L1, with two microphones and an acoustic guitar going through it. We do not use stage monitors at all, they're not needed when the PA is behind you! It takes some effort to get feedback.
 
For one very small gig I even ran my keyboards through the Bose to save lugging my amps up some stairs. Those little speakers shouldered the load OK, but the main failing of the L1s is lack of highs. Perfect for vocals, and acceptable for guitar, but too dull for piano.

Back on my first question I am going to experiment with the speakers I have placed behind me (elevated on a stand). I was assuming that the singers body in front of the mic may be a factor for cutting potential feedback. 
 

I have a buddy that is in a duo that uses two of these positioned behind them.  They usually don't have feedback issues.
 
The Bose L1 system, as I understand it, has some technology that eliminates feedback.  Trying this with a system that doesn't have similar technology may not work very well.

So goes the smart designers and not so smart - price differences follow 

Long before feedback detectors/busters, etc, I discovered ....maybe encountered is more accurate - something, on a gig. I'd built my PA speakers; EV used to have plans for a 12" Thiele cab. I used an EVM12L and I mounted Gollehan horns on top. Don't recall the crossovers. 
      I did a solo wedding gig in a small and very crowded room. The only speaker was right behind my head (with the horn above). I was using an EV RE-20 mic, which I loved (and miss). Anyway, it was very noisy and hot in the room. I dropped my pick and bent over to fetch it (tortoise shells were still legal then) and then through, "oh crap, it'll feedback!" But it didn't and my PA volume was "up" to compete with the gang!
     I "decided" the reason it didn't feedback was because the of the lack of spikes/smoothness of the mic and the speaker. So, there may be other ways/reasons that allow the PA behind you.
Michael

Feedback has a lot to do with the room as well as the mics, foldback and PA. Some rooms are great, others awful and the rooms which are best for unamplified speech are often the most troublesome for feedback.

The Bose L1 system is based "loosely" around line array speaker theory where multiple speakers stacked vertically propagate a cylindrical energy pattern. The reason for reduced feedback is simply due to the fact that the microphone hears only an average of the total energy emitted by the system. There is no clever technology involved, just basic physics, though to operate as a theoretical true line array, a system must be cleverly engineered.
There is a great deal of information to be found relating to line array principles on the internet. Just google line array.
Many manufacturers are now bringing out small systems suitable for small stage use based "loosely" on the theory. Not true line array, but effective.

Good to know what the technology is.  Thanks.