as a safety for subs would 80 to 100 hz inline crossovers be good to add to the amp... I actually turned my stereo up to almost 1/4 of the way last night and my subdidnt blow.. Also HU crossover set to 80hz amp crossover on LPF set to 80 hz .. Sony Sub recommended 18 to 2500 hz ..300 watts rms.1100 watts peak..

I have a 3 way infinity elec crossover I may use again.. not sure though.. we will see.

 

There's no need to add an in-line x-over. The amp's built in x-over is enough. If your sub blows at 1/4 volume, you've got problems. How much power are you running to it? What size sub do you have? It's better to have more power than not enough. Cause you can always turn the amp gain down.