I looked again at the Giandel site, and I see four standard 15A outlets, plus a hard wire connection for high power appliances, to the right of the four outlets. I don't know if you are planning to use this special connection, but it would be interesting if you do, to compare the sound of that to one of the 15A outlets. With your low total power draw at 220W max, any of the 15A outlets should be enough.
Also, they said that the Giandel is overbuilt with special EMF filtering. BINGO, that alone may explain the sonic superiority of the Giandel as heard by Oeno. I await your exciting experiments.
My concern is that Giandel advises to turn it off when not in use. I don't know why. This would be a problem with amps that need continuous low power for standby. The manual cautions that if the battery voltage drops below 9.5, the inverter will shut down. That doesn't leave much room for the draining battery starting at 12.6V, so you would have to keep the battery charger running all the time, disconnecting the charger for critical listening. Ricevs says the sound is not as high quality unless you disconnect the battery charging cables. See what you find if you keep the charging cables connected all the time, which would be convenient.