Tube maintenance for Audio research sp-16 pre-amp

I have had occasional issues with background noise with the ARC sp-16.

A few months back, after bringing in the Bel Canto amps curently in my system, I noticed a faint but noticeable and irritating high frequency whine or tone when the system is on and no music playing. There is virtually no background noise otehrwise, so when this low level high pitch noise occurs, it is subtle but clearly noticeable. I re-arranged the 6 stock 12AX7 tubes that came with the unit and found that changing tubes around affected the noise. Eventually, I hit the right combo that eliminated the noise completely.

Yesterday, however I noticed the noise had come back. After checking all wires connections and such I then re-arranged the tubes again. The first try made it worse, then switching two of the tubes eliminated the problem again.

I am considering acquiring a set of 6 new tubes to see if that works to eliminate the problem again and then keep the old tubes as spares.

Questions:

1) I like the current sound and am not looking to change it much but am interested in advice regarding what tubes to use and where to get them.

2) Does this ring a bell with any tube gurus out there? has anybody had similar issues? Is this kind of noise as I describe it a common issue? Is it an indication of a bad or at least lesser performing tube or tubes in the bunch?

I'm also open to any advice regarding how to best handle tube noise issues when they occur moving forward. I tend to not like to have to fiddle around with this kind of thing, so any advice on how to best avoid this kind of issue down the road is appreciated.