pre-amp unacceptable noise level?

How much is an unacceptable level of noise/humming etc. from a pre-amp? I just bought a new tube pre-amp and when I replaced my old solid state one with it the noise level from the speakers practically tripled. There is an audible humming from the listening chair 10ft. away. I can't say I'm particularly happy with this considering the one I replaced did not do this.