Basically, the PS2 will try to read the disc. If the laser is too weak to get the reading of the disc, the PS2 will stop, center the head again and will boost the laser power using the AGC (automatic gain control). That's why some PS2s can read discs after a few tries.
I'd suggest you to don't mess with the pots on the 400Q as they already come pre adjusted from the laser maker. It's just fit and play. All you have to bother with is the mechanical adjustments. You can use your osciloscope to figure out the best mechanical adjust.
I recommend a DVD-ROM game disc with a movie close to the edge of the disc (so you can adjust the skew screw) while you monitors the RF out from the laser
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)