apt holman problems

When is the last time that you inspected your ground connection between TT and preamp ? Wires from cartridge to headshell ? Interconnects from TT to preamp ?

If all of those are good, try checking the AC orientation of all of your components to make sure that they are correct. Here's a link to a previous thread. This can make a VERY big difference, especially on such low level signals like phono systems. I ran into this when i installed a TT with a "two pronged" preamp that lacked AC polarity on the plug. Simply reversing the AC plug got rid of it.

http://forum.audiogon.com/cgi-bin/fr.pl?htech&1014011751&openfrom&17&4#17

If all of that fails, try pulling your preamp apart and make sure that all internal screws holding the board down are tight. I would suggest backing them all out a few turns and then snugging them down rather than just snugging them down. This guarantees a fresh and clean "metal to metal" contact and may reduce / cure your hum based problem. Hope this helps... Sean
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I am not familiar with that Rega, but does that Rega turntable have built-in RCA cables?? I would also suspect a worn out cable, possibly at the RCA plug, especially if the cables are old. With the turntable on, try giggling the wires at the RCA plug and twisting the RCA plug and see if the hum goes away or gets worse. If so, replace the cables, or at least the RCA plugs on the end of the existing wires. Same goes for the ground wire as Sean suggests.

1st culprit I would be checking is the cartridge/cartridge wiring. You did not mention if you replaced it or not or what it was. And any of the other info in the 2 posts above should put you on solid ground, no pun intended till I had the thought.