Well, first they increase the stylus pressure slightly above the recommended setting. Then they make sure the TT's are not on the same table as the monitors. And finally they put the TT's on materials that vibrate less.
One (expensive) trick I learned on the Trancemission gig I played at is to use flat marblestone (perfectly flat!!!). You need three or four marblestones per TT (that's the expensive part). You put two stones parallel to each other, then on top of them the other two parallel, but crossed with the two first.
This symbol shows approximatively how to position the stones : #. The TT feet rest on the two top stones.
The result is flagrant : we spinned with 16 000 W RMS (speakers near the TT's) and no feedback at all!!!
There are certainly other ways to solve the problem, but those I mentioned are the most used. And anyway, there will always be a point that you'll get feedback whatever you do. But then you are probably using too high volumes!
