Mystery of the whistling amplifier

I replaced the manufacturer-supplied low-power tubes (two 12AU7 in the pre-amp section; four 12AT7 in the amp section) in my VTL integrated amp with NOS Telefunkens from Vintage Tube Services. After a few months I started hearing hissing and whistling through the speakers. It happens regardless of the source (DAC or the phono preamp).  I read here and elsewhere that this was probably caused by about-to-fail preamp tubes. Here're the things I've tried to address this:

1.   I took the 12AU7s out and switched them between their sockets. I also reseated them in their original positions. These measures made the problem go away for a while. Then it came back.

2.  I put the manufacturer-supplied tubes (JJ Elec ECC81) back in, and that "solved" the problem for a day or two; then it didn't.

3.  A Youtube video recommended scraping the pins shiny with an x-Acto knife blade. I did this, and it worked for a while. Doing it again, perhaps more thoroughly, also worked for a while, and seems to have reduced the overall level of the sound. I also hit the pins with Deoxit after scraping.

I listen to this setup several hours a day. Sometimes, like right now, there's no unwanted noise. When it is happening, you can't hear it through loud passages at all. But of course I want to fix this.

I'd welcome any suggestions or diagnoses before bringing this to the dealer.

Perhaps the tube sockets themselves need cleaning.  That may account for the amp sounding ok for a while after you swap tubes around.
Sounds like some oscillation somewhere. Check for any cold solder joints, or bad caps along the way. If the sockets are soldered into the PCB check that moving the sockets doesn't move the solder. :)
Sounds to me like you have microphonic tubes. Give the tubes a good twack with your index finger and see if they react (whistle changes or goes away temporarily). If they do, replace them.

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No, it's when they gave you a lobotomy, Lizzie. Or maybe you were born that way. Who knows?
Once the discussion swings around to lobotomies martinitime shows up. Coincidence? 
Once the discussion swings around to lobotomies, geoffkait is around. Even before maritime51 shows up. Coincidence? Trailblazing?
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Do not worry, we know you did not have a lobotomy. In fact, compared to a few other posters on other threads, you are quite a reasonable and jovial person devoid of narcissistic traits. I will stop short of claiming you are an asset. ET included.
You’re running on all four cylinders today, glubson! What’s got into you?! Excited that Santa is coming? 🎅🏻

12 cylinders. Do not underestimate me.

I wrote a letter to Santa. I asked him for a new bicycle pump for you.
Now that’s funny!  Never can tell when you’ll have a flat, Kitty. 😱
I thought more along the lines of applied physics that geoffkait indulges in.

Cute discussion starts on page 2. (from paragraph 2.2)
I'd buy another set of tubes and try them.  If they don't fix it, no worries, tubes don't last forever.   This sounds like it will be a trial and error fix since a lot of things can cause this.   Cracked solder joints can do this, so a reflow of the affected circuity is in order.  Check out the capacitors in the local area, as those are suspect given the symptoms.   Let's call that Plan A, Plan B is to take it to a reputable dealer or, better yet, back to the factory for a fix. 
Thanks to all who responded. The problem is "solved." I tried yet another set of J&J tubes in the pre-amp section and still had hissing and popping. So I decided to take the thing back to the dealer, and replaced all six NOS Telefunkens (two 12AU7 in the pre-amp and four 12AT7 in the amp section) with the original J&J tubes the unit had been delivered with. Hissing and popping went away, and I now have six rather expensive tubes on the shelf.

In truth I noticed a subtle improvement in sound with the original tubes. Not sure what to make of all this but again I'm grateful for the help and suggestions here.