Upgrade would be cosmetic only. Warranty is only part of the issue. After warranty service is a big issue, and a disgrace. I have had to junk a Marantz 7705 after five years due to no parts. I now have the 7706 which is virtually identical, including the SNR spec, but the 7706 is dead quiet, where the 7705 had very slight background. I think my 7705 was a "Monday morning" unit. If your current receiver is working fine, keep it, and don't risk getting a unit that may be trouble. The motto, "let what is good be good" applies here and my advice, and strong, is to abide by it.
Having to use an external amp is a bonus. The less amps in use from one power supply the better. More amps in the Cinema 40 is a downgrade.