Designing a variable band pass filter in the MHz range can be approached using tunable capacitors or inductors, which allow for adjustments to the pass band frequency. For more complex requirements, an IF-based system with an oscillator and mixer is a standard method used in receivers. The choice between using OP-AMPs or LC tuned circuits will depend on the specific design needs, including the number of cascaded stages required. Digital filters are also an option, offering programmability for variable pass bands. Consulting with a qualified engineer or conducting further research is advisable for tailored solutions.