I very much doubt that anything could be checked in a worthwhile way - heater open circuit isn't going to cause a problem with the amp (apart from not working at all) for instance.
You need a statement with the amp' when it is returned stating that it was sent UK to Germany, it was repaired and is being returned, and the total cost of the repair and return carriage was £X (or Euro). The main thing is that customs should not take insured value as what you have spent. There is probably a particualr customs form to be able to do this.
If it is picked-up by customs you will pay roughly, around, 30% of the declared costs in charges - VAT, duty plus handling charge. Getting caught by customs is not a certainty - I find about a third get missed, completely unlinked to parcel size, sender's country and value.