I've been playing my Mark Bass Little Mark 800 & MB Standard 104 HF cab and loving it. However at our last gig, (the first time that I have used my MB amp at this venue) whenever I played an F on the E string it sounded awful... really boomy. The open E sounded fine. For all of my other live gigs with the MB, I've been setting the EQ flat, the gain at 10:00 and volume to taste. The VLE & VPF are off. I am playing a Fender American Jazz 4 string with custom 60's pickups. String gauge is 105-45. Both pups are on full and the treble is a little less than 3/4. Any suggestion on EQing, etc?