You have to track down the source of it. Start off by isolating the preamp from ALL other components other than the amp. This means removing all of your interconnects and power cords while leaving the preamp hooked up to the power amp. If there is no buzz, start attaching one component at a time via the interconnects and power cords. Keep hooking up components one at a time and checking for the buzz after each one. Keep going until you find something that does generate the buzz.
For clarity sake, let's say that you've hooked up your tuner with no buzz. Then you hook up your tape deck with no buzz. Then you hook up your CD and the buzz is there. Here's where it gets tricky.
Once you've found that specific component ( in this case, the CD player), remove all the others from the circuit and leave it hooked to the preamp by itself. If it still buzzes, there is some type of problem occuring between that component ( the CD ) and the preamp / amp combo. You might be able to correct this by either reversing polarity on the power plugs ( if they are two pronged ) or lifting various grounds. Try various combos of the above.
If you've removed all of the other components and only left the buzzer ( CD ) hooked to the preamp with no more apparent hum, there is a problem occuring between the CD and another source component with the common connection being the preamp. Leave the buzzer ( CD ) hooked up and hook up the other sources one by one. By this, i mean the CD and tuner only. If no buzz, then try the CD and tape deck while removing the tuner. If no buzz, then remove the tape deck and try the CD and phono, etc... Do this until you find the two that are "fighting" with each other.
If you are able to find the problem this way, you once again have to start playing with power cord polarity or lifting grounds. This can be VERY tough and tedious work but it will be the only way to solve it short of replacing components. You may find that you end up with only ONE ground for the whole system. This is fine so long as you're using a power cord with a heavy conductor for the ground.
If after all of this, you still have a buzz and can't seem to get rid of it, narrow it down to which component is creating the problem. It's quite possible that everything will work fine until you install one specific piece ( the CD in this case ). Some components just will not work in specific systems and you end up having to replace it. I ran into this with one specific piece of gear and it drove me crazy. The really bad part about it was that it performed awesome in the system except for a low level hum. I ended up having to sacrifice that one source, which i was really happy with, in order to maintain the level of reproduction that i was able to assemble throughout the rest of the chain. Hopefully, you won't end up in that same boat. Sean
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