Assorted questions - MKIV

1: I'm thinking of putting JJ 6L6GC in my MKIV Rev A (aug '91). It arrived used with groove tubes. Can I expect it to be a safe change?

2: The V3 slot (labelled 12ax7 slot) doesn't like anything but a 12at7 it seems. Not that I'm complaining, it sounds good. If I put let's say a Tungsol 12ax7 there the amp makes a loud noise. Wrongly labelled?

3: I never use reverb and keep it on zero. How much does V4 affect the lead sound? Currently using a spax tube there.

Edit: Not entirely sure on JJ's though, do you have a suggestion for power tubes and more importantly pre amp tubes that are great for a tight metal tone and that are interchangeable i.e requires no tinkering with the amp?

The manual has the tube tasks/functions on page 17:
https://mesa-boogie.imgix.net/media/User%20Manuals/Mark%204.pdf

V1A= Input Stage
V1B= Tone Recovery Stage (post tone controls)
V2A= Third Gain Stage (Rhythm Modes) and Fifth Lead Gain Stage
V2B= FX Return Stage
V3A= First Lead High Gain Stage
V3B= Reverb Return
V4A= Second Lead High Gain Stage
V4B= Reverb Send
V5= Phase Inverter

There is also advice on tube noise on page 18 and how to diagnose. This video covers much the same process:
https://www.youtube.com/watch?v=wSl-l9aWDHw

Power tubes do need to fall in the correct bias range with Mesa amps(unless the amp has been modded). The easiest way to get the correct tubes is buy Mesa tubes -- they're ready to drop in.

Eurotubes sells tubes in the correct bias range and they often give you some choice of cold through to hot: https://www.eurotubes.com/store/pc/Retube%E2%84%A2-Kits-for-Mesa-c10.htm
If not send them an email and ask what they can do for you: [email protected]

There is also TAD: https://www.tubeampdoctor.com/en/tubeset-for-mesa-boogie-mk-iv-a
You''ll notice in the TAD tube set linked above for the MKIVa they offer a balanced 12AT7 for V4 (but not V3).

Some eBay sellers sell sets too: https://www.ebay.com/itm/191909939117?hash=item2caeb947ad:g2MAAOxygo9Q6DtY

Mesa only recommends Mesa tubes and at one time recommend GT tubes.
Lots of people sell tubes in the correct bias range if you ask.
Personally I don't use them because their idea of the correct range isn't always the same as what Mesa designed the amp to use.

For instance Eurotubes tends to like a hotter bias than recommended by Mesa.

Your amp isn't mislabeled. V3 is not designed to use a 12AT7.
Not all amps can switch out preamp tube types and not cause damage to the amp. Check with the amp manufacturer to know before switching out types.