Preamp inverts signal.

Fact is CDs do not have a mix of polarity, the vast majority are inverted. Lots of coverage of this online, the link below is to one of the most well known sites.  Changing polarity on my Paganini is a pain so I set it inverted for the majority, I try not to use the polarity change switch on the amp if I can as as another commentor pointed out it does change the active circuit. Just occasionally I find an LP that benefits however, for example yesterday on the "Breaking Glass" S/T (no great recording but inverting really helped)
http://www.absolutepolarity.com/

What is interesting and a little mysterious in George's very thorough explanation of absolute polarity is that entire CD LABEL repertoire are inverted polarity which seems to rule out randomness of CDs being inverted polarity. Mapleshade is an exception but on George's list the big boys like Mercury Living Presence and RCA Living Stereo CDs and Deutsches Gramophon are inverted polarity. So, ensuring one's system is polarity inverting might actually be a very good idea if his taste runs toward classical music. My only nagging reservation is that nobody else has confirmed George's list as far as I know, which does show an overwhelmingly high percentage of CDs having inverted polarity. I do find myself much preferring analog versions of RCA Living Stereo recordings including cassettes over the CD version, but it's difficult to say whether the reason fir this preference can be attributed entirely to the difference in absolute polarity, but I'm open to the idea that it might be a lot of the reason.

Phd,  I invert it at the amp side, makes no difference though 

Allrighty boys, today I’m swapping speaker leads and hope I don’t burn the house down ;) will report back in a few days. Wish me luck, going into the bush.

Sidebar, how can I test my system for polarity? Test track? 

"Sidebar, how can I test my system for polarity? Test track?"

The most reliable way to test the system for absolute polarity is the in phase and out of phase tracks of the XLO Test CD. Of your system is in the correct absolute polarity the Out of Phase track will sound like it is coming at you from all around the room with no specific direction. And conversely the in phase track will sound like it's coming from dead center and be very focused. These effects are much more pronounced with proper speaker set up and after careful attention to room tuning. 

Geoff Kait