How to speed up Mathematica replacement of matrix elements

Question

I have several 100x15 matrices; one of them is a distance. When elements of that matrix exceed a bound, I want to reset those elements to zero and also reset the corresponding elements of three other matrices to zero. Here's my silly way (but it works):

Do[ If[ xnow[[i, j]] > L, xnow[[i, j]] = 0.;
                  cellactvA[[i, j ]]  = 0.;
                  cellactvB[[i, j ]]  = 0.;
                  cellactvC[[i, j ]]  = 0.;   ], (* endIF  *)
   { i, 1, nstrips}, {j, 1, ncells}       ];  (* endDO *)

I tried ReplacePart:

 xnow = ReplacePart[ xnow, Position[ xnow, x_?(# > L &) ] ]

(something like this, I don't have it handy; it was done correctly enough to execute), but it was as slow as the loop and did not produce the correct replacement structure in matrix xnow. Please advise on how to do this in a way that is reasonably quick, as this calc is inside another loop (over time) that executes many many times. The overall calculation is of course, now, very slow. Thanks in advance.

---

Here is how I did this in R; very simple and quick:

    # -- find indices of cells outside window
indxoutRW  <- which( xnow > L, arr.ind=T )

    # -- reset cells outside window
cellrateA[indxoutRW] <- 0 
cellrateB[indxoutRW] <- 0 
cellrateC[indxoutRW] <- 0 

    # -- move reset cells back to left side
 xnow[indxoutRW]    <- xnow[indxoutRW] - L