I'm not a football expert, but the two DTs we've signed seem to excel in pass-rushing and not the opposite. Am I missing something here?
Jarran Reed and Dre'mont Jones are both very capable of playing a two gap DL role (such as 4i, a typical 3-4 DE alignment). What you are looking for in that role is a stout anchor often helped by a low center of gravity. Long arms are also beneficial (>33") which both players have. It's a sacrificial role where the lineman is often facing a double team and their job is not to lose too badly, so that the people behind them can win more easily.
I don't know that you are missing anything, but there are two possibilities to consider.
First, keep in mind that these two guys are only a piece of our eventual defensive line. We'll need to rotate them with others, and it's a lot easier to find rookies who are run specialists or pass specialists than it is to find prospects who are good at both. If we're going to sign FAs early then it's much better than they can contribute in both areas than if we had signed guys who are only situational run defenders.
Second, if you're looking at PFF grades then I would take them with a grain of salt. Football is a team game and it's tough for an outsider to properly grade somebody's contribution within the confines of the support they are getting from the scheme and teammates. Belichick has said that he wouldn't feel comfortable himself grading a defensive lineman on another team without knowing their exact responsibilities, and the guys PFF have doing grades are... well not in the same tier as Bill Belichick.
For an example, take a look at Roquan Smith, who is widely viewed by NFL personnel as one of the top off-ball linebackers. His PFF grades did not reflect that, as he was being asked to do everything in the Bears defense without a lot of help. When he moved to the Ravens defense this last season his PFF grades went way up, because suddenly the DL was a lot better in front of him and that let him look much better to the typical observer.
Essentially, PFF grades should be interpreted as "how productive does this guy look to an outside observer." If a player is not productive it might be their fault, or the scheme's fault, or their teammates fault, or the opponent's "fault." The PFF grade doesn't tell you which and really shouldn't be interpreted as a grade, but of course that's a better selling point than calling it a "productivity guess."