Never, never connect more than one TMCC or Legacy base to the layout ground rails. It may work but strange things happen. Your 30x60 should be fine with a single base. The RF signal from the base is broadcast in two paths: through the track ground rails, and through house wiring and copper pipes (The ground plane). Review Dale Manquen's TMCC Signal Basics article as a reference.
Loss of signal will probably be the result of poor ground plane and there are many threads in this forum on resolving ground plane issues. Some things I have learned helping to resolve TMCC signal issues on multi-level layouts:
1. Make sure the TMCC output is connected to each loop's ground rails.
2. Isolate all metal bridges and accessories from the ground rails.
3. Run a ground plane on the underside of all upper levels that overlay lower level tracks. Aluminum and copper tape are your friends here. Metal bridges (isolated from rails) are connected to the ground plane.
4. Frequently test the isolation of track rails from the ground plane. A single track screw or accessory connection can destroy the isolation.
Solve ground plane issues before adding a signal booster.