Amp advice needed

So I run a Pro-Ject RM5.1SE with a Sumiko Blue #2 cartridge through an Adcom GFP-555II preamp using the bypass output to preserve as much signal as possible. Amplification is two Yamaha P2075 amps in vertical bi-amp configuration to Polk RTI6.

I listen to mostly classic rock (Zeppelin, Kansas, Yes, etc.) as well as classical and some acoustic stuff.

I use high quality but not high end audioquest interconnects and speaker cable.

I'm really happy with the detail I'm hearing but some of it sounds a little thin. I know a lot of this gets real subjective real fast but I'm thinking I'd like a sound that is a little warmer and has a bit more bottom end.

What would you invest in first?
Different amps or add a sub?

I have an opportunity to purchase an Adcom GFA-2535 4-channel amp which I would use in the same configuration at a reasonable price. I thought it would be a nice match for my Adcom preamp and perhaps add some more of that warmth and bass.

Or, conversely, I could add a sub such as the PSW110.

Thoughts?