Don't think you're going to find anything comparable for that price with that kind of power. Quieting down the Korean guns is a problem, if noise is a primary issue, you might need to consider something else. It's also tough finding a repeater for that kind of money, although you might get close in a used gun. Some of the older Air Arms occasionally sell for around $600, but that's usually the single shot. My S400E is one of those, in .177 it will do 27 fpe with 16 gr. Eun Jins, and is very quiet. Also incredibly accurate. Oddly (?) enough, I don't think the .22 version is that much more powerful. If you NEED the raw power, you either buy a Korean gun and live with the noise, or spend LOTS more money :-). I believe a tricked out Air Force Condor from Talon Tunes would get you power and quiet, but in a single shot and for about 3X the bucks.
Guess I should have asked this first, what's your primary anticipated use? Hunting, back yard plinking, competition? There ARE rifles that can do it all, but if 90% of your shooting will be in the back yard at paper, there's no reason for a 60+ fpe .25. I recently realized that I have no real need for a 40+ fpe .22, so now I only have 2 rifles, both .177's. Granted, they're both at the top of the heap powerwise for a .177, and would be considered magnums if they were springers. But by zeroing in on what I really wanted/needed, I now have 2 world class rifles that will let me do it all. If freaking out the neighbors is an issue, I think you'd be better off starting with something else, rather than trying to make the Sumatra into something it was never meant to be. And it would probably cost less, too. Sorry if that wasn't much help.
Dave
