If you're tuning by ear I recommend turning it to where you think it's right, then backing it off some. Better safe than sorry if you aren't sure. An oscope is what shows clipping, a DMM just shows volts, amps, ohms, etc. Also your new subs are slightly beefier than your old ones, and will have a little more displacement which would raise tuning ever so slightly, not much but some.
How do you have your LPF and SSF set on the amp?