Toyota Tundra Question

This is my problem/question:

I have 2006 Toyota Tundra with the JBL system. I have replaced the factory head unit with a Kenwood unit (with the proper wire harness adapters to retain the factory amp controls and steering wheel controls); and I have added (2) 8” Kicker subs with a Sony mono amp. My issue is I think the system should sound better and I think the problem is the factory speakers. I don’t want to replace the speakers if it will not be a significant improvement. I’m thinking the factory amp is under powered for after-markets speakers. Is this the case or can I replace the stock 6.5 speakers with Polk or Infinity and realize a decent sound improvement?

Thanks,

Jim