You really need to have a long spring to know what that loud twang is. I've run AEG springs for 2 years before I ordered a proper length spring and the difference was night and day. You can check easily if your spring is too short, just fire the SVD without the dust cover and look at the spring. A correct length spring will stop moving really fast. A short spring will keep "swinging" for around 1 or 2 seconds and resonating. Some people have had success by spreading AEG springs to correct length to remove that sound, although it might decrease the spring durability.
For the piston I would recommend that you get their first gen piston. It takes standard AEG piston heads, which means you can use the Lee's precision air brake piston head to make it very silent (akin to a VSR with a Laylax piston).
http://www.leesprecision.com/collect...ke-piston-head
I've read only bad feedback from PPS parts compared to AirsoftPro. Ordering straight from AirsoftPro is cheaper than ordering the PPS parts from the states, unless some asian stores have started to carry them, which in the end could be cheaper than Airsoft Pro, I haven't looked for that recently.
Also, there is a thread on Airsoft Society with a lot of very good info on the A&K SVD:
http://www.airsoftsociety.com/forums...-thread-58026/