If I were to change out one component on my system, would I be better off changing the pre-amp or the amp to a tube device? ( or both )

Current system:

Pre Yamaha CX-1000 or Onkyo P-388F
Amp Yamaha MX-1000 and Onkyo M-508 ( bi-amped )

with Onkyo DX-788F cdp
and JBL S2600 speakers

Thanks for your insights and responses.

Tom