Those are nice speakers with a great looking veneer and that radio as center channel speaker stand is very cool.
As for the Onkyo settings, you can switch Volume display from Absolute to Relative using the onscreen Setup.
Do not confuse “Mono” Sound Mode with “Mono Music” Sound Mode. With “Mono Music,” all speakers will output the same sound in Mono at once.
When playing actual Mono signals, the Onkyo RZ50 allows for output to the Center Channel Speaker or the Front Speakers when using the “Mono” Sound Mode. Regardless of the output speakers, frequencies below the crossover setting of any speakers not set to Full Band will be sent to a connected subwoofer in “Mono” Sound Mode.
Using Direct mode, two channel signals will be output from the Front Speakers with no signal sent to a connected subwoofer and any and all room correction will be disabled. Onkyos do not feature LFE+Main subwoofer settings to get a signal to a subwoofer when using Direct mode.
Playing music in Direct mode at high volume can potentially damage the amps and/or speakers. Many damage their equipment chasing a bass dragon that cannot be caught when outputting sound from just the Front speakers.
In this particular setup, I wouldn't cross the Front and Center Speakers below 80Hz and I’d play with 100Hz to120Hz. Heights I’d do at 150Hz. Always make a capable subwoofer or subwoofers do the heavy lifting and take the literal heat off of the receiver and speakers.
Never use the Double Bass feature in Onkyo receivers when running Front Speakers Full Band to get a signal to the subwoofer. Those who want to compliment speakers set to Large/Full Band should get a Denon/Marantz unit and use LFE+Main.