Sunfire Signature... what is this?

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.

Looking at some rear panel photos that are available on the web, and at manuals that are available at the Sunfire website for what appear to be comparable later models, it appears that the 5A fuses (changed to 6A in later models) protect just the "current source" outputs, not the "voltage source" outputs.

My guess is that the 600W rating only applies to the voltage source outputs, and the CONTINUOUS output power capability of the current source outputs is far less (with the difference between the two outputs in terms of short-term (dynamic) power capability being much smaller). I suspect that the current source outputs are created by just putting the voltage source outputs through low value resistors having high power handling capability. The fuses are probably provided just to protect the resistors, with the voltage outputs being protected by circuitry.

So the 5A fuse rating, corresponding to 200W into 8 ohms, is probably understandable. The 10A main fuse does seem somewhat incongruous with the 600W/channel rating, as the 1200W AC input it would support (120V x 10A) would require that the amp be 100% efficient to output 2 x 600W continuously, which of course is not possible. Perhaps it's a design oversight, or perhaps the 600W continuous rating is a bit optimistic.

Regards,
-- Al

Not sure why you'd expect to hear much if any real difference. Most listening is at lower W's and very few peaks ever approach using all that headroom. As noted, the differences in your pre- and post-purchase amps is really kinda minimal (maybe 1.5dB if you could A/B 'em) so unless components have deteriorated and/or you're crankin' 'em all the way you'll likely have to imagine any big change.

Hmm.. ok, the reason I bought the 600 is that I had a 200x5 (C-1000), and although I didn't hear anything "wrong" with it, when I got the Cinema Grand 400x5 hooked up, there was a definite improvement (I know, different technology). So, I went with the "if more is better, still more is more better" philosophy.

It does make sense that there would be a point of diminishing returns. The 10-amp input, as Almarg notes, seems under-rated for 2x600w channels, and my speakers are 4-ohm (magnepan), so *could* pull 1200 watts each, which the amp wouldn't be able to supply for very long.

I hardly ever crank up the volume *really* high, but I again, at moderate levels, I hear an improvement between the
300w/ch C-1000 and 800w/ch Cinema Grand (into 4-ohms). I don't detect an improvement between the 800w and 1200w Sunfires, and maybe that's not to be expected, at moderate volume levels.

The "SS" mentioned by Rpfef appears at the end of the serial number, another clue that this is a Sunfire Signature.

I think I will send the amp to Rita's, and if I still hear no difference when I get it back, I'll figure out what to do, then. I'll have over $2k in it by then, and probably can't recover all of that, but I could then re-sell it with a clear conscience.

Thanks for the input. Great forum.