Just had the following response from UK Product Support, D&M Audiovisual Ltd. Hope this puts any concerns to rest.
"To date here in the UK, I as the UK support engineer for Marantz can confirm that to date I have not yet had a discussion with any member of the public about any physically faulty SR6004. This is the first time it's been mentioned to me at all, so as you can hopefully understand, I am very surprised at this burst of activity on some a/v forums. However you will find that not everything that is stated on the internet is truly factual, and a lot of it is blown out of proportion / fabricated maliciously, although there are some genuine cases.
We have had no faults reported here in the UK, and having spent some time checking through service site that I have access to, I cannot see any single occurance of an SR6004 ever having been reported.
Most physical symptoms (but certainly not all exclusively) with any AV receiver are created as a result of a poor setup / bad speaker matching / deployment of the receiver etc : there are many variables. But just to reassure you we are checking with the European Team currently, and will hear from them in the next day or two. If I hear anything further I will let you know."