Has anyone had success with a preamp with a Marantz PM 8006?

I agree with @tablejockey  however also believe assessment of the improvement to be expected relative to return on investment by purchase of an outboard phonostage would require knowing what TT, arm and cartridge is in use.

I previously had a PM 8006. It is known for having a good built in MM phono stage. I d experienced a nice improvement in vinyl playback by adding a Rothwell MCL step up transformer and using an MC cartridge. 

I currently have a Garrard 301 (Oil bearing) turntable with an SME 3009 non improved arm with a Ortofon 2m Brown Cartridge.

"Garrard 301 (Oil bearing) turntable with an SME 3009 non improved arm with a Ortofon 2m Brown Cartridge."

Spend more money and get a "better" main amp and phonostage.

The table setup warrants a substantial amplification upgrade.

@rpppr Again, I agree with @tablejockey.  Your TT set-up out classes the Marantz 8006. That said, with a vinyl system TT is the most important component. Regarding the purchase of an outboard phonostage, would you consider upgrading the integrated amplifier thereafter?