A point to remember about mandolins, and also guitars, is that the sound goes away from the player. Even if the player can't hear it there is a very good chance that everyone else can. Positioning a micro amp on the table in front of you and pointing at you might work if you can get agreement from your neighbours. On some amps headphones can cut out the speaker and judicious use of these can ameliorate the problems that come from sitting on the wrong side of an accordion player.