Hifi sound doesn't "fool" you. You problem is incorrect setup in a couple ways. As others have said the BETA58 is a super cardioid mic. You monitors should be at a 45 degree and on either side pointing towards the back of the mic (or your face). This will help reduce bleed as much as possible with that mic.
Also, a PA speaker and a guitar cab are just different beasts. Cabs do not have the extended frequency ranges of a PA speaker. You are hearing 2 different sources and getting 2 different sounds. Totally normal. You need to understand they will not be the same. You need to tweak at gig volume and with the band. Set up your patches as you see fit at home but finalize them at jam. A good band member will help out to get good tones live.
When you restricted the frequencies of the cab block you probably eliminated a specific frequency which was feeding back. This frequency could change in various rooms with various equipment. Keep this in mind! Good luck!