From what I gather, its an adjustment to help cut through the mix. Is this something that you only gauge correctly from the audience? Thanks, its always been the random know that I just leave around 4.
This is an obvious question, with all respect. Use your ears - either the tone will tell you to SUBDUE and SOFTEN (presence back) things or BRING THINGS OUT MORE WITH MORE DEFINITION (presence up). By it's definition, use it to give your amp the right amount of PRESENCE. You can guage it very well from right in front of your amp. It's no different a consideration than if your were adjusting your EQ, though it's not technically an EQ function. It's just a different tonal variable.
Everything said here sounds about perfect to me, all great advice and shows the presence can be very useful. The only thing I can add is that often in online amp reviews I've done I refer to presence as the 'room knob' adjust it to the size of the room you are playing. For a large room add more presence so everyone can hear you, in a studio its all tonal preference and gain/definition really. Same goes for amps with a 'cut' knob.
Presence seems to change the upper midrange IMHO. It is also useful for when you have a bright amp. For example, you can turn your treble down and raise the presence. I did this when I had a Fender Bassman reissue a guy on this forum suggested keeping the treble around 4 and turning the presence to 6 or 7. It worked really well and kept the unbroken in Jensens from killing my ears and still kept the sparkle without the ice pick.
Presence is a wierd 'function' on an amp. One JCM800 50 head I gigged with had SO much natural high end, I ran the treble and presence both on ZERO!
Also, since your everyday 12" speaker puts out frequencies up to ~4khz, it's probably important to understand at what frequency(ies) are being boosted by the amp's presence control. If you can hear what it does very clearly, it's probably around that same 4k range.
2.5-4k really adds 'definition' to live vocals and probably does the same for guitar. However, I still like to turn the amp up a bit and get that natural top-end compression from the tubes whenever possible. You'd probably hear more noticable benefit of a presence control at lower volumes than at full gig volume.
As others have said, think of the presence in concert with your treble knob. Look at them both as "regular highs" and "upper highs." Reach for the treb knob first, if you are looking for more definition. But if it you get "icepick," roll back the treb and bring up the Pres. Work them together.