H & k Grandmeister Deluxe 40 Squealing on 5 watt mode

[copperheadroads]

I got the Deluxe 40 here sorting out the power tube problem which has the tubes replaced & i checked & replaced the preamp tubes but this amp squeals when the volume on the guitar & the gain is about half way up on high gain setting , it only happens in 5 watt or 1 watt mode
20 & 40 sounds fine