I suspect a cracked solder joint in the speaker connection. If you have it set to stereo, does the channel work on the other set ? I always liked the 3400 - the most extravagant tuning dial ever seen!! Sounds better than the 6000 as well. 

Hi thanks for the fast reply, I have opened it up and put a voltmeter on the cables from the output plug that the speaker connects to. The wires go to (one of the caps and the other to the headphone socket)  I was getting a value on the volt meter  so it all seems to be connected as it should be. The second set of speakers is working absolutely fine as it should  do on both channels with no issues in both mono/stereo. I have tried different pairs of speakers as well so I know its not my S35's. I have also tried cutting the speaker output cables and re soldering them but its still the same so I am really stumped with it now, I can still use it but its more of an annoyance!  I was really shocked that its still on its original caps it looks like its never been opened up before since new!

Many Thanks,

Lewis

Check the speaker socket itself.
The contacts inside can suffer from fatigue and sometimes even break off and fall out.

Martin

Hi Martin, Sorry for my delay in replying. I have just purchased a new 2 pin din speaker socket and soldered it on to the existing wires and its still exactly the same so I am really stumped now with whats going on with it.

Lewis

All sorted, I actually sent the 3400 away for a full service it turns out it was a faulty headphone socket causing the issue!