Normally when using high volumes and your pickups feed back, I check to see if all the external components such as screws and pole pieces are firm. On older pickups, I don’t like to alter the pickups as potting and shielding can alter the value of the pickups. If the covers are on and the pickups are newer, I would use a vacuum wax potting done under complete supervision. At times I’ve used a small bit of silicone under the cover and filled in the pickup cavity with a foam rubber often found in pickup boxes. I try to fill all voids that could also cause the pickups to feedback and also raising the height of the pickup to get a better signal to output ratio.

Remember, altering old vintage pickups will reduce the value and collectibility. If you ever decide to sell the instrument, the buyer will most likely want the unaltered “stock” original pickups.