It is difficult to say how strong these players were. You can’t compare them with modern grandmasters. The modern grandmaster stands on the shoulders of these giants of the past. In the old times a lot of stuff needed to be invented. Steinitz himself was one of the first grandmasters who formulated basic chess principles.
These principles, which favored a more cautious and positional style, were not to every-bodies liking. Most of the (top) players in that era were still stuck in the Romantic way of playing chess. This meant:
Go for the attack and try to beat you opponent with great combinations.
Defense seemed not very important. Only a coward plays defensive moves! Here you have an example.
It is a game between Morphy and Anderssen. These masters were considered to be the top players of their time. And of course Morphy became legendry. There are still people who consider him the best player of all time. I am afraid that Morphy wouldn’t stand a chance even to an average grandmaster nowadays.
Of course this is not fair because modern chess masters (and amateurs) train hours and hours daily and can dip in a tremendous pool of chess knowledge. Just see how Anderssen’s defense play is (to say it politely) not of the best quality.