Audio Symphonies pre owners, tube questions

I recently purchased an Audio Symphonies Plus pre amp, and I have a couple quick questions about the tubes.

1.) The 3 input tubes come with metal caps over them. I contacted YS-Audio and all they told me was that they are used to help "quiet" the pre. I was wondering if anyone had noticed any diffference running the unit with them on, then off. Currently, I still have them on, but I would think that it would make some sort of difference (I know I could find out for myself by doing a listening test, but I figured I could get more info by asking here).

2.) I was told that replacing the 'stock' input tubes can make a dramatic difference in sound. I went ahead and purchased/installed some "New Groove", Mullard's before even turning the pre on. My question for this is, what about the 6x4 output stock tube? Should it be replaced too, and if so, what would be a decent brand to go with?

Thanks for your time,
The 6X4 is the rectifier, not output tube. I have NO idea if it has any affect on sound. The center 12AX7 is the 1st gain stage and is used in stereo. The left and right 'X7s are the 2nd gain stage and the cathode follower, separate for each channel.

I have some capacitor-upgrade notes I made when I had a Symphonies I can send you if you e-mail me at but substitute '@' for 'at'.
I'm no tube guru and have many more questions than answers. I think what you are referring to in the metal caps are tube shields. They, IME, are spring loaded and act to reduce microphonics. Beyong that they shield the tube from interference. Leave the tubes on and place your hand over the tubes. They will remain quiet...usually. Take them off and do the same and you will hear noise. If you want to stare at the glow of tubes in the dark then leave them off.

I'm of no help with your output tube question. I'm sure you will find a lot of help here on that subject. It may not have as much impact as changing the input tubes.
It is the tube sheilds that i'm refering to. Thanks for the input. I guess I'll leave them on.

I email you right now. Thanks!
So I guess I should also leave the rubber ring around the 6x4 tube on?
I have a symph+ and just put some GE JAN 5751s in the 2nd output stage and WOW !
big difference. especialy bass responce.

I want to try the GTs next.
I'm running 5751s in my Symphonies Plus: a Sylvania JHS Gold in the first gain stage & a matched pair of RCA black plates in the second; also have a GEC CV4005 as a rectifier. It's truly wonderful, clearly superior to my modded Acurus RL-11 in almost every way. The only thing is that the soundstage is a bit shrunken, but OTOH I'm only about half way through the recommended 100 hour burn-in period.
BTW, I took the tube shields off & installed UltraSonic HAL-O tube dampers...