tube amp/flutter in speakers?

i have a nice pair of mono blks that are making my woofers flutter. when i adjust the hum bal it gets stronger or softer depending on which way i turn the adj. my interconnects are good. i am using an old pair of ess monitors with the heil driver. the amps are sansui HF V60. rated at 15wpc i belive?. thanks to all who help.