Tubes and noise

[Ropet]

I play with VTL 6.5 preamp, which is a hybride amp with a pair of 12AU7 in the voltage section. The hardest challenge is to find low noise tubes each time it is time for change. The noise is volume independent and some tubes are really noisy and impossible to use. At this time I have tubes that are quite ok but not the best I've had. But now I start to think if it is another problem...

 

Today I took out my VTL MB-450 III and connected my "summer power amp" as I do not listen that critical during the summer and to avoid some tube heat in my room. What stuck me then is there is no noise of whatsoever. So now I suspect that the noise is coming from my power amps that amplify the groundnoise from my pre-amp in another way. As tubes are not for free I ask if any of you have any thoughts on where to start and what can be behind this? I'm just a music lover and want to rely on my gear.

 

The mono blocks has 8 tubes of 6550 each and one 12AT7 plus a 12BH7 each. Which one of these tubes are more likely to add to groundnoise together with 12AU7 in the preamp in your logic view?