Count me as one of those that thinks your current system using the HK990 will greatly benefit from an external phono stage right now. It will only make an LP played on the very good system you have now, sound even better!
Most externals offer far better performance and cartridge loading options than built-ins. And when you do upgrade your amp to something else, an external phono stage is portable and can go along for the ride.
You have been given many good suggestions so go for it.