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It's safe Megasam, and yes it's possible. Some equipment does reverse polarity, but the manual should tell you if that's the case. Otherwise there is a test for this on the XLO "test & burn in disk". There are probably other disks with this test, the XLO is the one I own so that's all I'm aware of. J.D.
If you reverse the leads on just one speaker, you will be running your system out of PHASE (unless one of the speakers is wired differently from the other). This should be very noticeable to you as a lack of image specificity--for example, a vocalist who previously appeared in the center of your soundstange may now sound like he or she is standing outside of one of the speakers.
When J.D. talks above about some equipment reversing POLARITY, he is referring to the polarity of both channels. If you reverse the leads (plus to minus, minus to plus) on BOTH speakers (or at the amp), you reverse the polarity. Some people hear this difference, others do not. Achieving correct polarity in your system can get complicated because, as J.D. says, some components (mainly preamps) reverse the polarity, so you need to compensate for this elsewhere in the system, such as by reversing the leads at the speakers. More than this, however, conventional wisdom says that about half of recordings do not have correct polarity. This is why some preamps and DACs have polarity inversion switches. These switches are much more useful when they come with a remote control. Dan
To positively insure your speakers are not phase reversed from the factory or after a repair, test them with a 1.5 volt dry cell battery. These are the same as used in a common household flashlight. Connect the plus terminal of the battery to the plus terminal of the speaker, and likewise with the minus terminal. The moment both terminals are connected, the woofer should make a very small forward excursion. If both speakers test the same, they are both in phase. The same test can be done through the speaker wire, to insure phase accuracy of wire and speaker together. Just connect the battery in place of the amp, with plus to plus as described above. This is useful if the positive and negative leads are enclosed within in a single housing, where you cannot visually verify that the color code or markings are correct.
I do not intend to plug an item I have for sale on the site...but if you REALLY want to be sure, the best way is to use the Sheffield Labs/LAX polarity tester and test track CD. Not only will it measure relative polarity, but it will also tell you if the absolute polarity of your system (or a given recording) is off. Fun and useful if you do a lot of component swapping..and very useful if you amp and/or DAC/CD Plate allow you to switch absolute polarity. Alas, my preamp don't do that, and i just sold my Muse 296, so it don't do it for me anymore.