Although some people would disagree to it's very existence, beaming is something you aren't really going to be able to properly tame. It's the nature a speaker cone to "beam" and people sell some nifty little gadgets that are supposed to fix the problem. Those gadgets may help you out a little bit, and as a matter of fact my friends old Matchless DC-30 has a similar beam blocking design built directly into the frame of the amplifier, but in the end you can't completely get rid of it. Especially at high volumes.
Marshall amps are just fine on the highs. Trust me, I've lost some of my hearing because of them ;). What's likely happening is that you are hearing the bass frequencies more due to the directionality of the speakers. Bass frequencies tend to roll off the outside of the cone and spread out at a wide angle, while all of the middle and high frequencies are being 'beamed' outward from the center at a much narrower angle relative to the axis of the cone. I have read articles where guys say this isn't how it works, but in the end I'll trust my ears first. Since the amplifier is at a lower elevation than your head, you're hearing the frequencies that more readily spread at a wider 'view angle.'** Stick your head down in front of the speaker (with the volume low mind you) and play a bit. You'll hear something 100% different that your usual stage mix. This, among many other more obvious reasons, is why people mike their cabinets and combos in isolation away from the stage. It reduces stage volume--which makes the engineer types happy--and allows for more of the true sound of the combo/cabinet picked up by your mike of choice to saturate the PA.
The ways I used to get around beaming (when I had to) were low volume + floor wedges or in-ear monitors with the amplifier isolated and properly miked--and really that's the industry standard way. Not many professional folks are playing with working half or full stacks on stage these days; it usually just serves to piss the sound engineer of proper. The problem with floor wedges is they can get really muddy, especially if you don't do a thorough sound check. In this case definitely work with the engineer at the venue to get a good mix that you're happy with. Ideally, you'd be using in-ear monitors with your own separate channel. There's really no better way to play once you get used to them. With wedges you have to stay glued to the 'view angle'** of your monitor or you loose yourself, and in-ears generally fix that problem. The only thing you have to worry about there is a dead battery :D.
If in-ears aren't an option, then when you're setting up for a gig ask whoever owns the equipment if you can isolate the amp and mike it. If they won't let you move it, then just turn it such that if faces away from the stage, mike it, get a proper monitor mix, and you're off to the races. Beaming will also kill your crowd's hearing--so this will do the naive kids that like to stand directly in front of very loud speakers a favor their mother will thank you for.
If miking and mixing to your own monitor mix isn't an option then you are in a less than ideal situation so simply make the best of it and rock on. Angling the cabinet can help, but it still won't get you as good a mix as wedges or in-ears could.
** You can think of the way that sound spreads from a speaker cone just like the view angle on an old projection television. As you move past, say 180 degrees, the picture goes away. Same think with a speaker. As you move past a certain angle relative to the center of the cone some frequencies will disappear.
