According to the CD Speed website the Nero CD-DVD Speed Disc Quality Test does not work with Plextor drives. If you want to perform PI/PO tests then you have to use either PlexTools Professional or PlexTools Professional XL.
BUT unfortunately the so-called Q-Check functions of the PlexTools software only work with:
-Premium (C1/C2 [CD-R/RW] error scanning)
-PX-712A (C1/C2 [CD-R/RW] and PI/PO [DVDR/RW] error scanning)
-PX-716A (C1/C2 [CD-R/RW] and PI/PO [DVDR/RW] error scanning)
So with your drives (PX-708A and PX-W4012TA) you cannot use these functions. There is nothing you can do about this unfortunately. Sorry..