Audio Symphonies pre owners, tube questions


Hi,
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,
Kevin
kld
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.