Unfortunately, I have no crossover data available so I was hoping that somebody here knows more. The speakers are old Altecs and I think this particular HF/LF combination is called Altec 608-16X.

That model number isn't jumping out at me.

The closest I can imaging is the Atlec 604 and that merely refers to the driver itself. The 604 is a coax (Altec called it a duplex) driver where the HF compression driver is mounted to the back of the 15" cone driver.

If you could give me the specific Altec drivers in the system (including your existing crossover), I can probably help you.

Steve

But the story gets more interesting . My friend says that he chose the 604s because of good past experiences with 604s, then with crossovers called "N809-8A", at a single-screen theatre now long gone. He also has a spare 604 speaker with the original filter in; that filter, according to him, is called "904-8A".

As to the crossover or "filter" as you term it. Altec calls all of their crossovers "networks" so all Altec "networks" begin with the letter "N". An N809-8A would be an 800Hz crossover with an 8-Ohms impedance as presented to the amplifier.

Now as to your 604 you need to determine just what the impedance of BOTH the cone and the diaphram are to ensure the crossover your connect will be correct. The impednace of the driver will interact with the reactive components of the crossover and affect the actual crossover frequency.

Since your crossover is an 800Hz based on it's model number...the horn of the 604 is completely unloaded below 1.2KHz and as such, you would have miserable response below there until the bass driver rolls in around 800Hz. Now, the 604 had a "multi-cell" type horn (6-cells) up and until the most recent model, the "K" version which has the Mantaray. In either case, I would keep the crossover at 1.5KHz. If you go too much higher, the 15 inch driver will start to lose response (a bit too massive for those higher frequncy).

Steve