You will need to have some sort of external EQ system to dial out the lows a bit more. I'm assuming you dialed the lows all the way down when you ran into the feedback issue when turning up.
I have used the S1 in just about every possible way, every position, every type of venue, with and without external preamp and EQ. I remember only once when I was first testing the S1 outside that I ran into what you ran into as I was playing it really loud with the volume dialed up past half, more like 3/4 or higher. I found I had to dial back the lows (bass) ALL the way out to tame the feedback. But that was also with signal preamped so it was already hotter.
For me, if I'm playing through a preamp or mixer, I also find that the S1 sounds best with the ToneMatch switches set to off. If I'm going straight in then I find the ToneMatch is best set to vocals for both vocals and guitar. We were practicing outside yesterday with the S1 in it's simplest form, just my guitar (Emerald X10 with active pickup, onboard volume and tone control) and my wife's mic. We had both ToneMatch switches set to mic/vocal and we were able to dial up to 3/4 and got a great, relatively loud sound with no distortion.
One thing you don't mention is what position you were using the Bose S1 in when you encountered the feedback. Pole, flat, or tilt back? I have found that the tilt back position is the most prone to low-end feedback but I usually only notice that indoors on hardwood floors. Try different positioning as well. Move yourself or the S1 to a different area. I'm always surprised when practicing inside at loud volumes just how sensitive feedback is to position. Just a step in a certain direction can increase or decrease feedback, so try that.
We recently just went to a second Bose S1 which we used as both a monitor or a second FOH depending on the situation. We had a QSC k8.2 as backup for when we needed more power but in the end we never used it and decided that a second S1 would be a much better addition and it indeed is. I primarily used an Emerald X10 with a Ghost bridge pickup systems these days and this is very less sensitive to feedback than my other Emeralds with K&K's but I wouldn't think a B-Band which is a UST would be sensitive either. Try decreasing your onboard volume when you increase the S1 and dial out the bass lows to nothing on the S1. Even with it dialed all the way out you'll still hear plenty of lows when the volume is cranked.
A small preamp or mixer would be very helpful. I typically go one of three ways. With my X10 we use an EAE StompMix X6 digital mixer for 'full gigs' where we have two mics and my guitar and I need full EQ, notch, compression and effects control in a relatively small and battery operated package. For smaller gigs lately I just go with the X10 and one mic straight into the S1 but I always carry with me the tiny and yet extremely powerful and useful Headway EDM-1 preamp. Runs on battery and has full EQ and a sweepable high pass filter that they call an anti-feedback switch which dials out those troublesome lows. It works extremely well an both the EDM-1 and the X6 are serious pieces of quality high-end kit.
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