He sounds like a great dog. But you are quite unsure of what he is capable of, and with a big powerful dog like that, you HAVE to know.
This is quite "natural" behaviour. He is protecting his territory and what he considers as his Pack and home, against "intruders". He sees the repairmen as intruders and a possible threat.
It's my opinion he is just one step removed from driving them away physically. He's like a thunderstorm waiting to break. That, you don't want. Ask the average reapirman if "a dog is allowed one bite" ! I bet he would say no.
OK he may NEVER bite.....but you can't take the risk.
I am not the behaviour expert here. All I can say to help is what I do with my own dog under such circumstances (she can be ever so slightly protective when repairmen come too, unless, and up to the time when they make an effort to make friends with her. But not all repairmen want to do that. They just want to get on with their job.)
As soon as the knock on the door comes, I make it clear to my dog that I am in charge, and move in front of her while saying calmly and firmly "Go get on your bed now". Then she does go to her bed. I say "Stay" I let the repairman in, at which point she might bark and try to get up again. But I give her the order to stay and wait on her bed, and she does that. I make it clear by my voice and my body-language, that I am in charge of the repairman's visit, and entering "our territory", and that he is allowed in.
She usually calms right down though can still look a bit wary, but she does as she's told and stays on her bed.
It also might be a very good idea (to really set your mind at rest and definitely prevent any unwanted 'incidents'....to buy a soft muzzle for your dog, and put that on during a visit by a stranger. It won't mean anything upsetting to your dog, even though you might feel it's over-cautious. He will get so used to the routine, and it is definitely better safe than sorry.
But don't let the muzzle replace the discipline. Do both.
Luke is a HUGE baby!! How do i know this? He is my "granddog"!! This is my daughters dog so everyone say hi!! sara