Acoustat servo amps ??

If the servos on the IVs are stock and the ESL panels are early generation vintage, then factor in the operational condition of the amps and tubes and I'd say you have a heck of a lot of variables and a wide performance span. Heck, the only way to sort that out is to go have a listen.

What vintage are your IIIs, anyway? --original coarse 3-wire, OFC-copper 3-wire with improved coating, or 5-wire version?

I will say that if the IVs are early vintage and the amps are stock then $1600 is probably top-dollar -- and that provided they are in good working order. You may be much better off picking up a good pair of updated/modified servos and using your present IIIs if they are at least the OFC-version panels.

BTW, Kubla, I'm not certain that you can use the servos correctly with Spectra-series panels, plus I'm not sure that Spectra panels are actually as good as the late 5-wire Acoustat panels.

Additionally, the servos come with different EQ curves depending on whether you are using stacked panels (e.g., 2+2s, 1+1s, or 6s) or non-stacked (1s, 2s, 3s). The bass EQ is increased to drive the taller models, I believe.

A passive preamp will drive stock servos OK, but then, the stock servos have adjustable input pots. Some modified servos have an input driver stage (and volume pot) removed and need a lot of input voltage -- these would not work well with passives but would sound better because the suboptimal active gain stage is deleted.

Well, yes, the servos can be great, but there are MANY different mods and levels of servos -- the best (and most lethal, literally) in my experience, are the AHT/Dan Fanny modified units that use the 6LB6 tubes and dual 7308 input drivers. Please note that high-voltage parts are sometimes hard to locate and these amps do require regular periodic maintenance to ensure top performance and to replace certain components BEFORE they disintegrate.

One advantage to the 6LB6 servos is that they can drive up to 8 panels per amp -- so if you own Acoustat Sixes or Eights, you don't need 2 amplifiers per speaker; one will do nicely.

Of the Acoustats I have owned the ones that I liked the best were 2+2s and IIIs. The Sixes are great with the AHT-modified servos if your listening room is VERY large (mine wans't big enough). They are capable of playing unbelieveably loud, with tremendous bass. A few of my friends own these systems/amps and are quite happy. :)

I have since moved on to the InnerSound Eros because it plays loudly and precisely, is smaller, and I no longer have to worry about dealing with the attention the servos require.

Kubla,

To determine which panels you have you need to look at the wires coming off the panels. If they are the very early panels, the stranded wire is relatively coarse and silver in color. If you have the OFC panels the wire will be fine copper stranded. The 5-wire panels use the same type of OFC stranded copper wire, but obviously there are 5 wires per panel instead of 3.

Lou, I never said that the model 3s were Spectras, so perhaps you are confused. BTW, The Spectra panels are the same as the late 5-wire non-spectra panels except that they have high-voltage resistors wired in to limit the high frequency reproduction to a more narrow section of the panel. In theory this seems like a plausible approach but I'm not sure how great it worked in actual practice. Some say the late non-Spectra panels are more transparent because the signal isn't being routed through resistors. So it would seem there is a trade off between transparency and dispersion at play.