My counterpoint developed some problem letting a little dc through intermittently, I believe it is a problem with the timer/relay on it. It didnt blow up. I have been using tubes for decades and have never blown up a tube anything. I have blown up a few solid state power amps however. Trouble with tube pres is usually when they begin to get complicated, like audio research, Conrad Johnson, and then they dont blow up, just some ancillary circuitry like a regulator or something goes south.
There should be nothing very complicated about the electronics in a preamp. The switching and the wiring to the switch is usually pretty imposing but the amplifier itself should be simple. There's no advantage to "complicated", if that's where the complication is, then it's probably poor design.
As for the hum - I would be very surprised if there was anything much wrong with the amplifier. Many of the part failures that lead to hum are not going to cause a *slight* hum and certainly won't lead to picking up faint radio signals. It's not at all unusual to have a low-level hum that is audible on high-efficiency speakers with the volume turned up. If it's a "buzz" then maybe there's an issue with the power supply caps or rectifier.
Hum is usually picked up from the inputs. The fact that its louder on the phono is pretty classic, since the overall gain through phono is obviously much higher than the line inputs. To see if this is true, put shorting plugs in the inputs (*not the outputs!*) and try again. If it goes away, it's being picked up by the input devices or improperly shielded cables. To test the cables, get the 79¢ RCA-brand cables from WalMart, Radio Shack, etc, and put those in place of the cables you have. If it still has hum, it's probably coming from the input device. If not, it's the "magic" cable, leave the inexpensive cables in place since they are equal to or superior to audiophile cables in any case. You can get fancy about tracking it down by shorting the far end of the cables, instead of at the connector, but it should be obvious pretty quick without that.
If the amp still hums with the inputs shorted, try moving the amplifier around, and reorienting it. I have a spot in my house where, for whatever reason, there's some magnetic field that creates a hum induced into the transformer. If I move it a foot to either side, or re-orient the offending device, it goes away.
Brett