Well here https://www.sweetwater.com/c455--Power_Amplifiers you can see a Behringer and a Crown around $300...dang COVID, there used to be something at like $165. Not sure about the features.Dayton Audio SPA250 plate amp, which is under $200. Are there external amps in that price range with the same features
GR Research has an open baffle version too!.I was going to recommend the Rythmik L12 as another prebuilt $500 option but just noted the price has gone up a bit since I bought 2 a couple years ago.
https://www.rythmikaudio.com/L12.html
I think there is something to their servo tech. The bass is musical and strong down to 20 Hz in my listening room.
cheers
chris