Agreed - the problem may be the monitors.

In our band we replaced big floor wedges with little monitor speakers (JBL Control 1's) on stands at head height and they work brilliantly. Our recorder/flute player said it was the first time she could really hear what she was playing. I'm more comfortable with my melodeon sound coming out of a small monitor close to me too. You need less overall sound and you can choose a position for the speaker to minimise feedback.

The speakers, amp and cable also pack up neatly into a small suitcase!

For a very loud band with electric guitars and drum kit small speakers might not be loud enough but in an environment like that I'd worry about my hearing and wonder if I was in the right band...

In-ear monitoring is the ultimate and expensive solution.

Preamps won't help.
EQ won't help much either unless the tone is out of balance and needs EQing anway, or if the feedback is way out of range of the instrument e.g. bass cut on a fiddle mic will kill a low frequency feedback but not affect the fiddle sound enough to matter. A feedback destroyer (self tuning notch filters) might gain you a few dB of headroom but may also make the sound wrong.

Anahata