Crackling and hiss - Preamp tubes not seated right?

I've posted here before about the cracking and hiss I get from my Primaluna Dialogue Premium. It became very noticeable during listening last week, so I took another stab at trying to isolate it.

When the amp is on, but with out anything playing, I can hear a low hiss and an intermittent crackle - the volume knob does not effect it.

It was only happening in the right speaker, when I swap the speakers it stays, so I figure it has the be the amp.

I swapped the three preamp tubes in 1,2 and 3 positions with the tubes in 4,5 and 6 position and it moved to the other speaker, so I assume it's a preamp tube problem.

I swapped out the 6 Mullard CV4003 for the original stock tubes, and it's silent. That felt good to finally get ahead of the problem.

I started to swap back in the Mullards one at a time to see when the 'bad' tube appeared. Swapping into position 1 and 2 were good, but it came back on when I added a tube to position 3, so I went back down to positions 1 and 2, and it was there again! But it was silent before? Very frustrating. I went back to all 6 of the stock tubes and it's silent again.

So my question is this - could it be that the Mullards don't seat as well as the stock tubes, and there is some arcing and other interference getting through? Should I give the pins and sockets a cleaning? Any advice on how to do that? Maybe I need an adapter to go into the socket to seat the Mullards?

I also think this may be the source of some distortion I was hearing, particularly on piano at certain higher frequencies. Does anyone have any thoughts on that?