I dont know about the low B string but I do know that the two new MB4210 Marshall I had were not very powerful for a 300 Watt amp, the first one went back as I thought it had a fault, got a replacement which was the same, sent that back and got an upgrade to a GK MB210 which was far better although more expensive .
Shame as the Marshall had nice tone but I suspect the Marshall had crap speakers as standard, the other thing with the Marshall was its weight, it damn near put my back out every time I lifted it !!
I see you are in Andover, I am in Basingstoke, if you want to try my GK out you are welcome, you can check your B string
