I have an Acdc prem and a LOTR (2004).Both with a external sub hooked up.So my question is in regards to a speaker upgrade.If i go with Flipper Fidelity speakers.Do i just replace the back box speakers due to already having a external sub?.Will i notice a difference?Or do i have to replace the internal sub also?And if i replace the internal sub then do i still use the external sub?