I've gotten started. (Koji: I didn't read your comments until a few minutes ago, so I haven't taken them into account yet). Attached is a first pass of the servo. It is missing some peripherals like the voltage supply and regulations, and I haven't even looked at the PCB layout yet, but it has the essentials of the design.

There is an input stage, followed by a linear-in-dB variable gain stage (which in light of Koji's comments might soon turn into a purely linear stage), then four independently switched filter stages, and finally an output stage, which can be inverted with a local switch. Between the last filter stage and the output stage are both a locally-switchable SWEEP input and a permanently connected EXC input. Koji suggests that we also make the EXC input switchable, though this should be done via remote. Note that the filter stage component values are currently just placeholders.

The boosts are all individually controlled via relay. The block of stuff in the middle of the schematic is one 4-channel analog comparator which will take our single analog "Boost Ctrl" input and resolve it into one of 5 possible scenarios (i.e. no boost, single, double, triple, quadruple). The comparator inputs have a range of -5 V to +3.9 V, so I've chosen the setting voltages to be -5, -3, -1, +1, +3 V. The outputs are 0 V or +5 V, so when the logic is 1, a voltage is applied to the base of the output's transistor and therefore to the relay. This is exactly how the remote boost is done on the universal servo, only now we can control 4 with one channel.

Please have a look and let me have your comments.

EDIT: I added the voltage regulators and the switch for the local sweep. Also, one of the connections to the inversion relay was wrong. I fixed it.