The 6500s were designed from the ground up to be similar to the CDT HDs, but are not identical. I've listened to HDs and classics, and I own 6500s, IMO the tweeter and xover on the 6500 set sounds a bit more balanced and musical.
Yes, they're made in the same buildhouse, and have very similar components, but they're definitely not identical to CDT HD series.
Just a tech note. You're better off getting the Nine.2 to power the 6500s over the Nine.4. NIne.2 will put out around 130 wrms/channel given healty electrical system, as all eD amps are rated @ 12 volts. A Nine.4 bridged will be overpowering the 6500s @ close to 200 watts/channel, and could very easily damage them.