Subwoofer Amplifier Repair Help!?

The brown substance looks more like over-heated glue than anything else, was it hard? - if so, it's supposed to be there, and it commonly turns brown due to age and heat exposure.

If it was liquid, then it might have been leakage from one of the capacitors? - but from the picture my first impression is just glue.

My suggestion would be to change the capacitors. The voltage and capacitance of the capacitors will is written on them. You can use higher voltage and / or larger capacitance. Make sure you match the polarity.

The diodes are probably OK. If you have a multimeter you can test them, as they should conduct one way and not the other, but you will need to unsolder one end before testing them.

If the loud noise was 100 Hz hum, then the bulk capacitors have deteriorated. Get same or equivalent high RMS rated cap. I agree, change all caps. e.g. https://www.digikey.ca/en/products/detail/rubycon/35ZLH1000MEFCG412-5X20/3568159 lowest ESR for this price