I own the Boss MT-2... The tone is extremely thrashy, but i only use it in front of my 15 watt... it sounds sweet in front of my half stack. Its really versitile. But go with the Metal Muff. I actually wish i bought the Metal Muff instead.
There arent many amps out there that can have just as good tone quite as loud, valve amps least of all... But Marshall has something called the Power Brake, from what I heard about it it's exactly what your looking for.