I know there are some fragments found in Bergen, clearly from bows. They are pinewood, probably pine compression wood. I know people with more info on theese fragments than I do, but they are quite well known. Also, there are some of the viking graves that were opened before good methods of preserving the wooden objects came along that have descriptions of bows. The oseberg grave contained at least one bow from what I've heard, probably made from elm. Unfortunately the only surviving evidence is the notes made by those who opened the grave..
The saga litterature is another source. What is certain from reading the sagas, is the fact that they competed in having a bow others were unable to draw. Knowing that gives a clue to believe heavy bows was quite common. The fact that those bows found suggests quite heavy bows passing 100#, combined with my personal guess that the hedeby bow wasn't the heaviest bow ever used by a viking, makes me believe they had some bows
at least in the same league as the MR bows. (I like to believe even heavier than that, but that's also coloured by me being a Norwegian..
)
-But, reading about Eirik Tambarskjelve in the sagas suggests that I more than possible may be right.
Another clue to believing the poundage were respectable, is all the sources of the average strenght amongst the vikings. They often Competed in different games and sports, and as far as I know there have never been any shortage of food. In fact sources suggests a very healthy way of living, and combined with all the physical work in their everyday life I believe the average man back then would laugh at the average man today..