Tweeter quest

I agree with Gregm, matching the tweeter to the other drivers is very important ... I am not sure what the effect is called but "speed" is a very good word for it...in essence the match affects how instruments with some HF harmonics sound to the ear. An instrument will have a tangible sounding timbre when this is well matched...almost real. If poorly matched then the instrument feels less tangible or real....although this depends on the listeners experience with the timbre of real instruments.

Yes I think they need to match - I was looking at one of the hivi isoplanar ribbons perhaps as they are more like conventional tweeters in shape and dispersion.

Rather than replace rhe Vifa tweeter you might be better of supplementing it with a ribbon used as a semi-super tweeter.I have found this works very well,especially with the 19mm Vifa tweeters.You can just sit the ribbon on top of the speaker and use a 0.5 mF cap to protect it and roll it off below 12000hz[approximately if it is 8 ohm].I would be hesitant to use anything of higher value than 0.5 mF .There is some risk of damage with first order crossover below this point,although I have used 1 mF on my Fountek JP2 without problems.

JT

Here is a site you may want to look at:

http://www.zaphaudio.com/nondomes/

He does a lot of driver testing and is pretty well respected in terms of his knowledge of drivers and DIY designs. Be prepared though, he's not of the "audiophile" camp and pretty much wants to attibute what he hears to what he measures. That can irritate some folks. Still, knowledge is valuable and he does present some interesting data.

I'd be careful with tweeter swaps. There are a few things working in your favor. I don't know the PMC's very well, but if they are truly active to all drivers with no passive components you might stand a chance of this working out. Not sure how much tailoring is possible. Also, your highish x-over point is working in your favor as ribbons get ever more unhappy the lower you cross them.

The big hurdle will be making this mod look halfway decent. You are going to have to move the tweeter center up because you can't center them in the current hole or you'd need to cut into the midrange driver. Moving them up means some trickier woodworking will be needed to make good round holes (or rectangular) in the new locations.

Also, since you are now increasing the center-to-center spacing you will change some of the interaction with the midrange. This may or may not mess with the resulting sound. (That's the reason they cut into the midrange flange in the first place when they designed these things. The higher the x-over point the closer together the drivers need to be for good integration.)

Go with a raven r3 and shit can the rest and start over.This would yield a better sounding loudspeaker than adding a ribbon to a loudspeaker designed for a diferant transducer. Ribbons sound wonderful but system needs to be design as a whole. Not just a add on.So maybe sell the PMC and DIY your own system based a r3. Would work very well with a 10-12in hieff woofer as a 2 way.Founteks neo pro 5i is a great ribbon almost as good as r3 but can run into the midrange and is 102db 1 watt. It also costs less.If you search about on tube amps or my adds. I have a 5i based system pictured maybe it will spark you into DIY.I have used most of the ribbons you mentioned and many others raven and 5i are the best I have heard.