Most power amps have a DC blocking capacitor at the input so that DC is kept out. However, since capacitors are considerd a "bad thing", some manufacturers do it differently. It sounds like your phono stage has DC on its output and should be used only with power amps that will cope with it. It's easy to spot which ones will be suitable if there's a circuit diagram to peruse, but not otherwise.
BTW, a phonostage straight into a power is unusual, isn't it?