please help preamp tubes not shining

Hi I purchased the Eclipse preamp new in 2011 and tonight after listening with pleasure as usual for half an hour the left chanel sound becomes softer and blur. So I turned it off and then waited for 2 minutes then turn on again.The sound becomes normal but I notice that the two tubes in front were not shining. so I switched the front 2 tubes with the back tubes. sound normal for both chanel but the front 2 tubes were not shining.Fuses are ok. red light both lit.sound ok. The problem is front 2 tubes not shining like they re not working at all but both chanel sound ok. Could anyonea help me indentify what is wrong with it?

Thanks in advance