I just want to gloat. : )

[voice of the theater]

No, if anything, others used theirs as design guides.

I don't recall a duplex loaded Heathkit and my memory of the details of the various furniture styles they offered has all but vanished except for the early '70's contemporary looking one with the lattice grill (the last model offered?), though I recall a Mediterranean style somewhere along the line, but this is such a rare loading scheme in what would be an extensive rework of an existing cab that one has to wonder why a DIYer would bother since for a HIFI app it works well enough as a simple BR.

Combined with getting these from a local theater and you confirming it as externally a factory cab, this strongly implies a Heathkit design, so for now these will remain an enigma with no real vintage value beyond the drivers.
[/SIZE][/B]GM

Until reading GM's post from Saturday, I had never heard of "double bass reflex tuning" or George Augsburger and had no idea this was that intricate of a design. I would have to agree that we seem to have a real mystery here -- if these do turn out to be modified AS-101's, then why would someone go through all that effort to modify the interior of the cabs using a DBR tuning design when "for a HIFI app it works well enough as a simple BR."

I found on another forum a guy who described how to bypass the Lpad, I was wondering if I might try the same thing? I keep it cranked up all the way anyways... The cutouts are 17 3/4 by no more than 4 inches high. Right now the speakers are being driven by an Optimus STA7500 receiver.

In one of your photos, I could see that the length of the port looks to be close to the right size for an 811b (from your measurements it turns out it's about an inch shorter) but you can't see the height of the port in the photo. Now that you've measured the port cutout as being only 4 inches high, if this is a modified AS-101, then someone must have fabricated a new baffle (as GM suggested as a possibility) because 4 inches is not enough room for an 811B horn. If someone went to all the trouble to build a new baffle (and had the know-how to incorporate DBR tuning), then perhaps these cabs are already appropriately tuned/ported as it seems unlikely that someone would go through the effort to incorporate a DBR design and then randomly decide on 17 3/4" by 4" as the port size.

I would recommend upgrading your Optimus receiver. Vintage Altecs like the ones you bought are very revealing of any weak links in your system (they can reveal lower quality components as sounding poor and can reveal higher quality components as sounding amazing). Even if the Optimus is still operating up to original specs, 604's sound SO much better with better amplification. In the meantime, as a test/experiment, try connecting your right speaker to the left channel of the receiver and your left speaker to the right channel of the receiver to see if reversing your channels has an impact on how the two speakers sound compared to each other. If you do this reversal, and notice that now the difference in sound quality isn't as great or that now the 604E sounds better than the 604C etc., then you know that part of the difference you're hearing is the receiver. Older (and especially) lower quality amps/receivers, can often have or develop problems with the two channels sounding different from each other. Combine that with two speakers/crossovers that have inherent differences, and that can really intensify the problem if you have the better sounding speaker hooked up to the better sounding channel of the receiver...... If there is a difference in the sound quality of your right and left channels of the receiver, upgrading your receiver may make it easier to get the speakers to sound more similar to each other by using the L-Pad adjustment (or bypassing the L Pad with or without a fixed resistor), and by recapping the crossovers (bad caps can really detract from the sound quality), and as GM suggested check all wiring and connectors. Capacitors aren't that expensive-- (Solens, Daytons, etc.) unless you buy boutique caps, so if you have $20 invested in these speakers and you spend a few dollars on capacitors and try all the above suggestions and the 604E still sounds considerably worse than the 604C, you're still way ahead of the game if you end up having to send the 604E to GPA for a recone/remag/check-up and they may charge you a hundred something dollars to make that 604E "sing" again. You still have next to nothing invested in a pair of speakers that with some TLC will hopefully sound amazing soon.....

BTW, can you post a couple of close-up photos of each crossover so that we can be sure if it's the right crossover for each duplex driver?