Just to confirm I understand:
1. When you connect your preamp to the Willsenton R8 Pre IN, you get substantial hum.
2. And when you connect to the R8 CD IN, it is silent.
The only difference between inputs on the R8 is that the Pre IN bypasses the volume control – ALL tubes are in circuit – even the preamp tubes.
So most likely you either have a ground loop issue on the Pre IN or you have a circuit issue on the Pre IN, the reason for this diagnosis is simple, if the Preamp was generating hum/noise you would easily hear it on the CD IN.
One simple test is to hook the Preamp up to the CD IN, and slowly bring the volume to max – if that volume level only produces a very low level hiss/noise then you know for sure the problem is in the R8 Pre. Why, because the CD IN and Pre IN will share the same ground return, so it must be a circuit problem in the R8.
I would just use the CD IN and call it good.
Truly,
Jim