Buzz or hum

I have a hum in my system, Will Vincent amp, PS audio preamp with gain. and Coincident speakers.The buzz does not go up with the volume. When I unplug the RCAs from the preamp that go into the amp the buzz stops. I also plugged in my CD player directly in the amp and there was no buzz. So it is somewhat isolated to the preamp, I guess?
Someone said I might have a grounding issue but how would I fix that?

More precisely I would say it's a grounding issue that involves your preamp, but it shouldn't necessarily be blamed on the preamp. These problems are vexing, but try the following. Clean both ends of your RCA interconnect and all the jacks they plug into (on the preamp and the amp). If you don't have a purpose-made cleaner, dip Q-tips in alcohol but don't drip excess alcohol anywhere, wipe off the metal contacts, then let the contacts dry. Make sure the shield is fitting snugly when it's plugged in. Still noisy? Then wiggle the jacks gently while powered up to see if you can make the hum stop and start. If you can, that suggests a loose solder joint. I've had this kind of problem when an interconnect wasn't seated firmly.

If the interconnect doesn't seem to be at fault, next look into interactions between the power supplies in the amp and preamp. Try plugging them into different outlets not on the same circuit. Make sure your outlets all have solid ground connections (there are testers you can buy at the Home Depot type store, if you aren't comfortable with turning off the circuit breakers and examining your outlet wiring - or get an electrician in). If you do find some causal chain in the outlets, it might be a good time to have an electrician install dedicated 20 amp outlets for your audio gear - it will help the sound.
Sounds like a ground loop hum. Many threads on that topic here. Search the archives.
Float the ground in the preamp (those little 3 prong adapters)...not a big deal. My preamp/amp combo used to hum and that's how I solved it until I bought a different preamp (Kavent S-33) and it's weirdly 2 prong. I also use the ground lift switch on my REL sub and it works fine.
It does sound like a ground loop or 60 HZ hum. Chassis grounding usually solves this problem.
I love these threads it gives me the opportunity to use my old adage, again. " why did the amp hum, because it didn't no the words ".
I have to mention (well...I do) that I scored a NOS PS Audio Humbuster. A great thing that works exactly as described for mechanical hummage. That's right...hummage.