I own a CAD 300SEI. Purchased new 4 years ago for $5K+ with all the options. You could contact Billy Wright CEO at Cary and provide the serial numbers. He will tell you the age and what options are included billy@caryaudio.com which should allow you to put a price on the units.
I love this little unit. I run it with Western Electric 300Bs into Klipsch Belles, Zu Dirty Weekends and Sennheiser headphones. It’s a keeper!