Best ways to reduce engine noise with car audio?

I've found that deadening material on the firewall and engine bay does way more than anything audio-side - it physically blocks vibrations before they enter the cabin. After that, a decent head unit with good noise isolation in the doors and proper speaker mounting makes the biggest difference since you're competing with engine rumble, not trying to eliminate it entirely.

One thing that doesn't get mentioned much is engine cover quality - a lot of people have flimsy plastic covers that rattle and transmit noise straight up, so upgrading to a proper fitted cover or even adding extra damping underneath it can cut down on what reaches your cabin before it becomes an audio problem. Combined with what's already been said about firewall treatment, focusing on the engine bay itself rather than chasing it with speakers and insulation after the fact saves you money and actually works better.