I have a Moon Audio Integrated 340i PX configured for phono output that I pair with a Rega 1. Curious what if anything I would gain by adding a phono preamp? Am I understanding correctly that this integrated model has a preamp already built-in?Your amp offered a phono section as an option. So do you have one or not? Your statement above kind of contradicts itself?
If you are currently using your Rega with it, I guess you have one then. Another phono stage may or may not offer any significant improvement. Also, upgrades seldom result in huge leaps in performance but rather smaller, incremental steps. Its this last 5% in performance that we all seem to seek. So more info would really be needed to give you any kind of meaningful answer. And then only your ears would be the ultimate judge.
For all we know, your may have the ideal phono stage already for your TT and the rest of your system.