I've found running a sub from a tape monitor output to be more trobulesome than it's worth.

This is before the volume control so every time you adjust the volume, you'll need to readjust the subs volume.

With a pre out, you generally set the subs volume once and from there on, it follows the volume settings on the receiver.

Of course, if you don't mind making two adjustments every time you change the volume, it's fine.

FWIW, by using speaker level inputs to a sub, it DOES low pass the low freqs to the sub AND high passes the remainder to the main. Most subs do this.

If one wishes to only avail themselves of the low pass capabilities, they simply need not connect the subs outputs and run the mains either from another parallel speaker terminal on the amp or, at line level, by using a "Y" connector at the preamps output and routing one side to the sub and the other to the mains amp. This is what I do with my maggies.