I purchased a KD550 in 1978 (ish). I replaced the tonearm with a Black Widow (carbon fiber version) and a Pickering 3000. I’ve never seriously considered replacing it, In recent years I’ve had the BW rewired, replaced the dust cover (notoriously weak hinges) with a nice 3rd party lift off and installed a Grado Platinum cartridge. It’s aged very well with my 20+ years of flying for USAF ears.
I almost added a KD650 three years ago; shipping was going to be too expensive.
Depending on where you live, I’d be interested in discussing your turntable. I live in the Hampton Roads area of Virginia. Please send me a PM if you are relatively close by.