Weight bearing on each pan, pan diameter, pan spacing and pan adjustment are the key factors relative to soil penetration.
Most of the Disc Harrow weight is in the pans. A Disc Harrow with 18" diameter pans is too light to reasonably break hard soil. A Disc Harrow with 18" diameter pans requires multiple passes to smooth recently plowed ground even under optimum conditions.
A Disc Harrow with 20" diameter pans is the first weight Disc Harrow with enough weight bearing on the pans to penetrate reasonably well.
An MX with cab should pull a TPH Tandem Disc Harrow with
22" diameter pans covering rear tire tracks through all soil types.
A Tandem Disc Harrow with 24" diameter pans cuts very well, though may be beyond the capability of your cabbed MX to pull at adequate speed.
Disc Harrow gang adjustment:
https://www.tractorbynet.com/forums...ing-three-point-hitch-mounted.html?highlight=
Generally speaking, working moist soil, a Tandem Disc Harrow with adequate weight will penetrate to half the disc diameter, less the diameter of the thick pan hub at the center of each pan. Pan hub diameter varies somewhat between pan manufacturers but usually 3" to 4".
Therefore a Tandem Disc Harrow with pans 22" in diameter, properly adjusted, working moist soil should penetrate to 7" depth on the second pass over plowed ground, smoothing as it goes.