I have now tried following:
Test 1-4: Sub 1 pre-out in use
- Surround amp off
Surround speaker set to Small
Crossover 100Hz
Result: Still noise in sub when setting level for rear surround channel
- Surround amp off and disconnect all cables to power amp
Surround speaker set to Small
Crossover 100Hz
Result: Still noise in sub when setting level for rear surround channel
- Surround amp off
Surround speakers set to Large
Subwoofer: LFE
Result: No sound I sub
- Surround amp off
Surround speakers set to Large
Subwoofer: LFE+Main
Result: Still noise in sub when setting level for rear surround channel
Test 5- : Sub 2 pre-out in use
- Surround amp off
Surround speaker set to Small
Crossover 100Hz
Result: No sound I sub
Something is wrong with the sub 1 pre-out. Since I run the sub signal via a minidsp I don’t really need both 1 and 2 pre-out so I can live with this issue. I could try to have it fixed by a local guy I know but it’s always a risk something else goes wrong, so I think I just let it be.