The new recommendation is to try mounting the sensors directly to the table centered directly below the feet of the bridge with the film end of the sensors near or touching the feet. Thus one sensor will be above the bass bar while the other will be above the sound post. This new location will pickup all motion of the table. The result is a much "fuller" low end response while still maintaining clarity in the midrange and high end. It is also noticeable less sensitive to string noise. This mounting location has the added advantage that it will work for smaller bridge structures including Cellos.
Currently one of the more popular pickups (Gage) uses a single sensor that mounts under one foot of the bridge, however, ore experiments indicate that both sensors contribute to the sound quality although the lower frequencies will come primarily from the sensor over the sound post.
Some musicians will have some concern attaching anything to the table of the instrument, however, we have found the 3M Removable Poster Tape to be gentle with most finishes and it removes without leaving an adhesive residue.
This sensor location be more prone to feedback problems than the bridge mounting location. This is not a fault of the pickup. Rather the instrument top and body are designed to resonate and reinforce the very low frequencies so the instrument body will tend to pick up sound from the speakers. If you need the highest possible gain before feedback, then the bridge mounting will probably work better for you.