Functionally, two 2200uF caps (C307/C308) and D302/303 is that of a half-wave voltage-doubler negative supply. It's fed by the ground-referenced 5.6VAC that runs the bulbs, and produces (about) a -12v output. To do this, C307 is charged on the positive half of the cycle, when D302 is forward-biased, and D303 is reverse-biased. As the input voltage descends, D302 becomes reverse-biased, and C307 passes its charge to C308 through D303, which in practice becomes forward-biased near the negative peak of the input AC. This setup is really inefficient, requires oversized capacitors, and makes very poor use of a transformer's current capability . . . but it's a cute way to get a separate -12v from the lamp winding. The only real function it serves is to power the "normal/limit" indicator lamps - the relay startup delay is separate, rectified by D301 with R301/303 and C301 forming the time-constant. There's also a switch formed by Q304, whereby the +15v that runs the meter circuit is turned on after the -12v indicator-lamp supply comes up . . . this is presumably to keep the meters from jumping around on startup.
As far as your issue goes . . . if it's C307 that blew up, this could also be caused by something drawing too way too much current on the little -12v supply . . . but I'm not seeing any likely "standard" failure modes that would cause this. Generally, electrolytics blowing up usually means that either it or its associated rectifier diode(s) were installed backwards.
From your photos, it looks like you have the caps in correctly, but it's not clear which way the (blown-up) diodes were installed. Also, there's a misprint in the manual for your version, pertaining to the switch for the meter +15v . . . only D306 is fed from the little -12v supply, while R309 and the emitter of Q304 are actually fed from the output of the 15v regulator (emitter Q302).
The first thing I would do would be to leave the blown-up parts in place, and run the amp without the meter card installed . . . and check to see if the +15v and -15v regulators come up and work correctly. Then check/replace the diodes/caps for the little -12v supply, and make sure it comes up properly . . . then check that Q304 is working correctly by verifying the same voltage exists on the collector and emitter, and that there's about 0.6v between its emitter and base.
After the power-supply is running, then remove the ICs from their sockets in the meter card, and put the meter card in place. Verify that the power-supply is still working correctly, and that there's indeed +15v and -15v getting to the right places on the meter card. Then . . . install the ICs, and again, re-verify the supply voltages.
It's important to go through the diagnostic process step by step . . . and NOT to replace parts in various different parts of the amplifier until you solve the basic issue. Otherwise, you can't separate whether you're looking for something that's the root cause of the original problem, or a possible mistake when changing parts.
