The TX-SV525 should have no problem driving 6 Ohm speakers. I bought one new in 1996 and only a few years ago gave it to my brother-in-law for his garage system. It is still in mint condition and performing flawlessly. The 525 is the middle of the line for that series. You can get the manual for it on the Onkyo website under support->downloads.
That receiver is ProLogic only. Unless you are getting it really cheap, I would consider stepping up to a Dolby Digital receiver. You can get a TX-SR502 (which I also have) brand new for about $200 and change.