Sunfire Signature... what is this?

The needle goes nearly to the 500... 450+ for sure, but I read that there was an adjustment pot for the meter on a board in the amp. I have a Sunfire Cinema Grand (405x5) that has 6-amp MDQ-6 fuses on each channel, which also leads me to suspect this amp is not 600w/channel.

If it says Signature on the back and has his signature on the front face as you say then it is surely a Sig 600. He did not make a Sig 300 etc....

If the binding posts have the red and black plastic covers, then it is a first generation. If the solid steel posts with an IEC, then the second version.

The 600 is very powerful and you should notice this immediately in your system.

I wasn't sure how much difference I would hear, going from one Sunfire at 800w/ch to another at 1200w/ch (4-ohm speakers).

The lack of difference was what made me start wondering.

All the manuals I can find on-line show 15-amp AC input fuse for both the 300 and 600 watt Sunfire 2-ch amps. Since this is a '99 model, maybe the 1st generation was different. And maybe I should send it to Rita's, for a tune-up.

Yes you are 100 percent right. Send it in and it will come back better then new. I could not believe the improvement. Also get an iec and use a better power cord.

Yours is the Sig 600 and I understand now that you already had lots of power before the 600.

The word "Signature" on the serial number sticker and the 480 Joule meter reading indicate pretty conclusively that you have the real 600 watt version. Also, it is likely your serial number is preceded by the designation "SS." This is yet another reliable indicator.
Carver has been profligate in post sale autographing. He'll sign pretty much anything he designed, so his signature on the front has come to mean very little.
Telling the difference between 400W and 600W is pretty close to impossible, all else being equal. This is only about a 1.5db difference, after all.
All of Carver's Sunfire "Load Invariant" designs use fundamentally the same circuit, with more or fewer output devices. They all sound practically identical, which is to say they all sound terrific.