That said and given your explanation of our defense, why is it that we don't see many tackles by our DB's at, behind or near the LOS? I just get the impression that whether there are 4 or 5 of them out there, they are so hell bent on trying to play pass defense against the 3-4 guys that might be running routes, that we are left with not enough guys near the LOS to limit run plays to short yardage, unless a LB is freed up.
At a high level, DBs shouldn't be responsible for making tackles at the LOS all that often. Maybe a safety depending on any particular play. Most NFL defenses are single-gap defenses. The linemen often align in a gap and they are responsible for runs to that gap. The linebackers are filled in based on offensive and defensive alignment. An easy example, in a 4-3, you often have a one DT as a 1 technique, one in a 3. The 1, outside shoulder of the center, is considered to be in one A gap. The middle linebacker, Mike, is normally responsible for the other A gap. The 3 technique is aligned on the outside shoulder of the guard and is considered to be responsible for that B gap.
To stop the run against base offense, you're hoping your Mike or your Will can scrape fast enough to be able stop the run with only 7 men in the box. It does require the defense to play well because they have 8 people to account for: 5 linemen, 1 TE, 1 FB, and the ball carrier. This is why defenses will bring the SS into the box on obvious running down and distances: 1 man to 1 man against the run. And in turn, this is why quarterback runs work as well as they do--they break run blocking math. Of course the risk it getting the QB hurt, but its all a trade off.
Going further down the rabbit hole, every defense aspires to rushing 4 and dropping 7. The reality is you need to play games in order to win, but it helps to start with the basics.
In every offensive scheme, there are 5 eligible receivers (exception: if the quarterback is shotgun, he can also catch a pass, but normally there is trickery happening and a different eligible receiver is now throwing). To run Man-2, ie, man to man w/ safeties deep, both CBs and all three linebackers have a man. In almost all situations, 1WR = 1CB, leaving the linebackers to deal with the HB, FB, and TE.
Cover-2 or Tampa-2 requires all 7 to drop in zone. The safeties have deep halves, just like in Man-2, the corners have the flats, the linebackers have the medium under-zones, 1/3 each.
In cover-3, the FS and CBs each have a deep 3rd, the SS and the linebackers have the medium zones, 1/4 each.
Cover-4 has the FS, SS, and CBs each with a deep quarter (ergo the secondary name, quarters) and the linebackers have the medium zones, 1/3 each.
If you're following along, Cover-N tells you how many "safeties" the coverage has. And you can tell a lot about what is probably going to happen on any play by finding the safeties and then watching if/how the roll after the ball is snapped.
Cover-1, single high safety, is usually a man coverage and 1 extra rusher or a spy.
Cover-0 is straight man and you're rushing 6. Extremely high risk--if the offense can block it well enough, someone is going to get open. If can be as easy take snap, take one step, ball is out. The defense is hoping to disrupt the patterns and have that ball fall harmlessly to the turf, but if you have a player that can win off the line (think D. Adams) and a quarterback who can get the ball out on target and on time, the defense has lost before the ball is even snapped.
Regardless of the coverage being ran, the secondary can elect to press, play "regular", or play off. Cover-2 normally wants to press to help cover up the natural holes in the zones. Seattle happily pressed out of cover-3 w/the LOB, though now we're getting into the weeds about how coverage is ran.
