1. What is the tractor brand / tractor model you will be using?
The tractor's weight and drive, 2-WD or 4-WD, will control the subsoiler you can pull.
2. As you have a Disc Plow for Primary Tillage, consider an All Purpose Plow for SecondaryTillage.
An APP will penetrate 12" to 14" deep.
Dirt Dog All Purpose Plow / Field Cultivator Dirt Dog Model APP 66-5 Total Width 66", Working Width 62", Five Tines, Weight 284 Pounds, Paid $835.00 (2010) Field Cultivator: A Three Point Hitch mounted, secondary tillage implement, used to perform operations such as soil aeration...
www.tractorbynet.com
3. Though you have a Disc Plow, consider a Moldboard Plow rather than a Subsoiler.
A 16" Moldboard Plow will sever roots 9" below the surface if your tractor has enough traction to pull one. For safe plowing of roots you will need a trip plow or a shear bolt protected plow to make sure your tractor will not flip over backwards.