To upgrade from simaudio 3.3i to 600i or not?

Volume-wise you won't get much more out of the 700i; the difference is from 125W/channel (600i) to 175W/channel (700i). And of course ecah time you double the watts it produces only a 3db increase. With only a difference of 50 watts/channel between the two models, the volume difference will less than a 1.5db increase.

The difference in sound quality, then, should be the main factor that determines whether you fork out another 4 grand or not. And sorry, although I've heard both the 600i and 700i (with focal 1028 BE / siltec 770 series / moon 750D), I haven't compared the two amps side by side.

I have, however, compared the Moon 330i next to the 700i. I say this only to point out that perhaps you are better off investing in two LessLoss DFPC Signatures to power your source and 600i. Why? Because I found that the performance enhancement offered by the LessLoss DFPC Signatures was clearly greater than the performance enhancement offered from upgrading from the 330i to the 700i. Not to mention, two Signatures would cost around $2100 (shipped from the EU), almost half of what it would cost you to bump up to the 700i. If you don't own LessLoss DFPCs already, I expect you'll be surprised how much just two such power cords will improve your system.

Harris4crna. I don't know how much 15% of your budget is, but 15% or not you owe it to yourself to order and listen to the LessLoss DFPC Signatures (one for your source, and one for your moon 600i). I've had the Signatures on the moon 700i and 750D (at the same time). It's a little scary how much the sound improves. You simply can't go wrong.

Nice speakers. And so you know, if you try the DFPC Signatures, you're likely to notice improved articulation in the base, among other things. Do let us know how things go, whatever you happen to choose.