amplifier hum

I am rather new to Audiogon. I am using a Sonographe SA 120 power amp and that I can hear it hum slightly. I was told that it is normal for amplifiers with large transformers to hum a bit. My concern is that if I listen closely to my speaker's woofer, I can also hear the hum coming out of its woofer and my tweeter makes a slight buzz too. The hum/buzz is reduced if I disconnect my pre-amp which is a McCormak micro line drive (used as a passive pre-amp.) My interconnect cables are quite short at less than 1 meter and I keep them away from power cords.

My amp uses a 2 prong power cord so I am thinking that it is not a ground problem. The amp requires 220 VAC so it is plugged in to a power conditioner which is in turn plugged to a 1000 watt step up transformer.

Is this hum/buzz coming through my speakers caused by RFI or is there something wrong with my amp?

To what extent will the step up transformer affect the sound quality? I don't listen to music that loud, up to 12 o'clock at the most in my volume pot so I'm thnking that my amp won't be needing that much current. I listen primarily to cd's and use a Rotel cd player connected to a basic MSB link Dac III.

I will highly appreciate your comments or suggestions.