Not heard the Cary, always have been curious about it though.
I own a Chardonnay and love it. It is point to point wired inside, can't see how it would break down easily. The hum may have been a ground loop issue which can be common in any setup. I have compared mine to a high end Belles preamp and Modwright and kept the Supratek. I did have one problem. The plastic power button got a crack in it and I was not able to power it on. I sent an email to Supratek and I soon received a replacement power switch. Good to know they support them, and this was when the company "shut doors". I soldered it in and I was good to go. Do need to consider that they are in Australia so if something does go wrong, a local tech could fix or send it back which would be expensive.
I have upgraded the caps in the preamp section and power supply to go effect but now we are getting into DIY stuff and I know you are considering stock. The new Chardoneys are suppose to be based on their previous preamp a step up from it. So I'd bet it would be killer. From what I have read about the Cary it seems like it would be a warmer sounding preamp. The nice thing about the Supratek is it can be molded to your taste with different tubes, and it takes a wide range of types.