Hi Dave,
If you check the "Product Museum" section of the Accuphase official site (under the Product section) you will see that Accuphase is releasing new models every 4 years (sometimes 5 years in the case of the top of the line models). For example the C-2800 was released in 07-2002 and was replaced by the C-2810 in 06.2006 which in turn was replaced by the C-2820 in 12.2011 (I believe). The C-2400 was released in 08.2003, so you can do the math :).
I did not realize that the C-28XX models do not have bass and treble control. Why are the bass and treble controls so important for you? (For me the balance knob it the important one :). )
Also, I did not realize there is a large difference between the C-2410 and C-2810 as Greg has mentioned. Most often people say that it is definitively not worth buying the C-2810 at almost twice the price since sound wise the two are very very similar. However, Greg's experience definitively correlates with the price difference between the two.
I have listen only the C-2810 model so I can not say much about this.
What speakers do you have?
Regards,
Paul