The 400 Pro is an excellent way to filter wall power. Unlike other line conditioners it offers a little moment to moment reserve power for amplifiers.The 400 Pro does a great job of cleaning high frequency noise which is so critical for DAC clocks to hold their accuracy. It is also important for preamps. This unit was used by a manufacturer in the lab to always have the best sound for product evaluation. We are installing a 1000 lb isolation transformer due to our horrible local industrial power so we are not needing the 400 Pro. If you are not able to upgrade your wall wiring the 400 Pro is an excellent solution to get cleaner mids and highs. For amplifiers I plug the 400 Pro in the wall and also plug the amplifier in the wall in the same outlet. The 400 Pro is in the class of conditioners that does "parallel filtering" so the lowest impedance is achieved at the outlet. ( a good thing). For front ends I plug the 400 Pro in the wall and the front end component into the 400 Pro. This divorces the front end from the wall for a slightly quieter connection. the 400 Pros can be "star clustered" (meaning multiple 400 Pros in the same outlet). This is beneficial if you have a big amplifier. One 400 Pro will not make much difference for a big amp. I have used 2 and 3 of them at various times where they saved my life at audio shows (really bad hotel power on 100 ft+ wall wiring runs, ugh). This one is in good condition. There are no age-related parts inside so a used one is as good as a new one. I have two of these. When one sells I will let the other. Thanks for looking.