Does anyone know the difference between SP, CH and the blank racquets, like is there a distinguished difference in quality, durability, ect? Thanks.
This thread in the FAQ section should answer your question. http://www.badmintoncentral.com/forums/showthread.php?t=10231 The "blank" you are refering to could be the low end entry level racket or some old models that do not carry the distribution region code. Nowadays, even the fake high-end Yonex rackets do have the serial number and production date and distribution codes well replicated.
You mean the differences in quality among the difference distribution region codes? There has always been a "conspiracy theory" that Yonex produces better quality racket for 'JP' code (for distribution in local Japan market) and the others are sub-standard. But there has not been any evidence to prove this or otherwise. But no, personally I donot think there is any differences in quality whether it is a SP, CH or JP code. One thing you need to know though is that because of the targeted players in the respective regions, the weight and grip size ranges for the rackets could be restricted. For example, it is much easier to find 4U rackets for SP, whilst 3U is more common say in the European or North America region codes.
The major difference is the racket warranty. You can only get the warranty replacement within the code country.