Effects racks and amp racks come in different depths. Some amps, GK and Hartke for example, are not as deep as some Ampeg heads and will fit nicely in an effects rack. My SVT 6 PRO will only fit in an amp rack.
"If you don't want the truth don't ask. Make up your own like everyone else does". (Michael Pare as Eddie Wilson/Joe West in Eddie and The Cruisers II).