rear firing tweeters....

I have a pair of old Snells with rear firing tweeters...just curious why we don't see this configuration more today ? Phase issues? Placement problems? Engineering issues?
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I've always wondered about the "correctness" of rear firing tweeters as well, but I have heard several that sound good, so I guess that's what counts.
I had a pait of Nestorovic 5As for a couple of years, and they had an optional and removeable rear firing tweeter module on top. I always preferred it on.
Who is we ??
Some of the Revels use them; I am sure there are others. As I sit IN FRONT of my speakers I have never noticed a need for them! Just kidding.
I have a pair of VS2 Von Scheickerts which do have an "ambience tweeter/?high midrange " driver. It does see to add space to the sound but not much more so than a good tube output power amp qould ( I use both). It doesn't seem to hurt that's for sure. My main speakers, in a different rig, have better 3D halography (Focals) and don't have them. From that little experience I conclude they are not needed in all cases, the VS may be making up for a particular issue of their own.
I do like that the Von Schweikert rear firing tweeters on the VR-4 and other models have attenuators so that you can adjust the level to better integrate the speaker in your listening space (or your listening preferences).
My satellites have rear-firing tweeters as well as the front ones. It certainly adds to the spacial qualities of the speaker system, but the stated goal of the designer is to even out the in-room high frequency response.
I had those Snells. E-IV, I believe? I liked them, but not necessarily because of the ambience tweeters. I think they were just good, high-value speakers.

To me, it seems a rear-firing tweeter is basically a reverb unit. There will be some delayed arrival at the ears, delayed enough to not cause phasing errors, but perceived more as an "echo". Ultimately, I'm not really comfortable with intentionally smearing the HF response in time to add "ambiance" beyond what is captured on the recording (or not).
The goal, at least for the Revels, is to smooth the polar response.
Sonus Faber seems to like the idea.
The explanation I heard was to balance out the speaker's power response such that the tonal balance of the room reflected sound better matched the direct sound at the listening position. A rear firing tweeter is not the only way to achieve this goal.
I meant VR-2 my mistake and apology. It does have a volume potentiometer as mentioned above, but it's functional range is very limited and on the low side.
My speakers fire front and rear 100% of the audio frequencies...
No problem.
Magnepan 3.6s..
Your Maggies are dipoles. Not better, not worse, per se, but certainly different than a pair of tweeters separated by 12" or so and firing in or out of phase.

One nice thing about dipoles is the null at 90 degrees, which can reduce sidewall effects. Everything is a tradeoff. I just don't necessarily buy the idea that a rear-firing tweeter solves enough problems to offset those it causes, but of course, we're talking about engineering here, which is as much a balance of compromise as anything else.

Nice speakers, those Maggie 3.6!
>I have a pair of old Snells with rear firing tweeters...just curious why we don't see this configuration more today ? Phase issues? Placement problems? Engineering issues?

We're trying for uniform polar response which is already much broader than the midrange's dispersion with typical mid-bass driver sizes (5"+) and cross-over points (2.5KHz+) where the local directivity minima contrasts with the mid-range driver's increasing directivity to produce a harsh sound due to excess energy in the 2-4 KHz range.

If you have mid-range output headed out back as in a dipole having similar polar response at higher frequencies helps and a rear firing tweeter works well; although otherwise it's not a good idea.
Failed to mention...I also own eosone by polk rear firing mains in my ht set I own 2 pairs of rear firing speakers...maybe more than I need ...ha...
A common denominator in much of this is design engineer Kevin Voecks, who designed the original Mirage line all the way into the beginning of the M-1 bipolar. Early Voecks designs at Mirage also had rear-firing drivers. Then he moved on to Snell (actually worked at both Snell and Mirage simultaneously for awhile) after Peter Snell died, and he redesigned their line, incorporating the rear-firing tweeters. He has been director of engineering at Revel since 1997.

There are other companies that have used ambience tweeters as well, and it's hard to say how many arrived at this on their own or were inspired by Voecks.

I've had Mirage M5si's for 16 years. In my 2-channel LR system I use a pair of forward-biased omnidirectional Mirage OMD-15's. After getting married in my living room with live music and soon after hearing a pair of Mirage speakers with the Omniguide, I became hooked on this approach to energize a listening space with the same timbres, room energy, and soundstage of live players.

If you want to hear just what's on the recording, no more no less, sit in the narrow sweet spot with nearfield monitors. If you want something that sounds like live music filling your listening area, go with omnis, bi-polars, or at least something with ambient drivers such as the rear-firing tweeter.
I don't mind what my listening room sounds like, but I do mind being told what live music sounds like since I've successfully played it and mixed it for money most of my life. Rear firing tweeters are interesting, sometimes sound great, but are simply unnecessary if your main speakers aren't surrounded by mattresses or 6 miles of open space.
"If you want to hear just what's on the recording, no more no less, sit in the narrow sweet spot with nearfield monitors. If you want something that sounds like live music filling your listening area, go with omnis, bi-polars, or at least something with ambient drivers such as the rear-firing tweeter."

I think that's a bit much. There are plenty of full-range systems which can extract what's in the recording w/o requiring nearfield seating and which don't need "ambience" tweeters. This doesn't need to devolve into a religous argument.
My Vandersteen 5A's have rear fireing tweeters which provide extra high frequency energy for "problem" rooms...however, it also presents phase problem, so they have an "off" position for most setups.
I have Von Schweikert VR4JRs. I have a small room which is 17 feet wide at the front and 24 feet deep with my sitting position half-way back. Ceilings are 9 feet. The walls are treated with deep pile quilts behind and to the side of the speakers which are 30" from the front wall. I have found that on large scale orchestral pieces that the rear tweeter helps "enlarge" the performance, thus better simulating the event. For small venue performances and more intimate spaces the rear tweeter is not necessary. It is only a tool to help the room recreate the music. I believe it is more about the room than the speakers themselves. There's not much to argue about here. To each his own. I would not buy a speaker because of the rear tweeter but I wouldn't reject a speaker because it has one. My speakers project the sonic image of the performers very well with excellent height, width and depth. The rear tweeter is not trying to make up for deficiencies in the speaker design, it is giving the listener options in tuning the music to the room.
It may be artificial...but rear firing drivers seem to created an enhanced firm of depth......these spatial ques might not be for everybody
I have always wondered about the benefits and have never heard an a-b comparison. Maybe the question is -- At a particular price point, do you want some of your money invested in rear firing tweeters vs having that money spent on some other aspect of the speaker. The 'less is more' argument, particularly if they are usually switched off.
Remember the Bose speakers that were designed to point 8 little EQd woofers (each) at the wall with one or two pointed at the listener? People loved those things...and then Bose took the front speakers off and turned them around to sell as Pro Sound speakers. About a zillion people bought those...and for nearfield PA stuff they're not too bad with proper EQ, although I think a good 2 way horn design is better. A lot of Bozes on that bus.
The reason there are rear-firing tweeters has to do with human hearing/perceptual rules.

When the ear hears a sound, the brain makes a copy of the sound and then looks for sounds that might be similar. If it finds one, it does a comparison and looks for differences. This is part of the brain's processing to sort out where the sound is coming from.

Essentially what is happening here is the speaker designer is taking advantage of the brain's processing by providing short-delay echo information. The result is that you get improved imaging and soundstage, because the brain is better able to sort out from where the sounds appears to be coming. So its not *just* for better polar response!
With my Revel Ultima Gems, Kevin Voecks (former head of design for Snell, who designed the O.P.'s speakers, and who is now head of Revel and brought with him his rear firing tweeter design to Revel) the purpose of the rear tweeter is to add a sense of "spaciousness" or "air". If your room is excessively damped, turning up the rear will be beneficial because otherwise, your room will prevent you from hearing the "spaciousness" from the recording. It absolutely works.

On the other hand, if your room has alot of hard surfaces and is highly reflective, turning the rear tweeter to "O" or turning it off does the trick becuase no boost is needed to create the "air". The level of the front firing tweeter and rear firing tweeter can be adjusted as well. This is an outstanding option as it allows you to adjust your speakers to your room and makes the speakers adjustable to a very broad range of room configuations. All speakers should have this design IMHO.