No Power no lights come on and no sign of any power

An internal component has failed. Without it in front of me for testing I cannot say which. You need to takje it to the repair shop.

I would try another ac adapter. Plug in that one and stick it on your tounge and see if it bites. It should only be 9-12 volts, just to see if it works. It may have a fuse inside but I doubt it, if you have to replace the adapter, there is a diagram on the back below the plug showing polarity and specs. Voltage must be the same and mA must be as specified or higher. NOT lower except for checking for lights or signs of power. Normal usage would need its requirements or higher. Make sure the + and - are correct to avoid damage.

The most likely case is that one of the main output transistors (if which there are many) is shorted. When the protect circuit senses DC on the final output, it will not allow the output relay to come on to protect the speakers from raw DC. With the power removed, you can check between each outputs Emitter and Collector and you will probably find close to zero ohms. From that point you need to unsolder each of the pins of the bad channel until you find the shorted device(s) and replace them.

Power supply. Most likely an electrolytic is failing. It may or may not show any signs of it. Other possibilities are bad solder connections, loose wire connections, or possibly a bad MOSFET if it uses a switchmode power supply. Also check the general construction to make sure nothing is shorting internally. Sometimes wires get pinched.

Check the bridge rectifier diodes for an open condition. Also check to see if the filter capacitor is heating up- if it is, replace the cap as it is shorted and the reason for no power. It may show signs of leakage already.

The floppy drives of this age often required strapping to select the unit number and other setups. Newer drives were not compatible. Make sure the drive selected light remains on when power is on. If not, check the drive selections jumpers for the drive.

The phantom power light wouldn't illuminate unless the unit was powered on. Possibly the main power indicator light is burned out.

A BARE USB cable should NOT have caused any problem. If it was an "Active" type cable that does a conversion something might have happened. Another possibility is you had a static charge on you and the static did damage. Another possibility is if the connector really wasn't the SAME type and pins were shorted. Turn off the power and with a bright light compare the connectors to see if this last possibility might be the case. Also look for signs of arcing or bent pins. If the connector was forced, then it is possible the circuit board was cracked.. In any event servicing is required.

Search the web for a "Factory Reset" sequence for the unit. Usually you have to hold a button or a combination as you turn power on.

You'll have to remove the amplifier section from the speaker cabinet and look inside the chassis.
Check the power transformer, the output transformer and the rectifier, see if any of these shows signs of overheating.