There was an era when you couldn't present a commercial control room to
the record business without a pair of 604's hanging above the window.
What made them a "studio monitor" in addition to a hifi speaker was
their durability and efficiency.
Bass players routinely pull jacks out of their Fender P-Basses when
they don't realize they are live - generating heart stopping blasts and
hums. After everyone gets up off the floor and apologies are made, the
Altec 604E's would still be working.
A few years later Audiotechnics proferred the Masterlab crossover
which extended the lifespan of the 604 as the default large speaker
studio monitor. Then, UREI time aligned them and released the 813, 811
etc, which was actually a 604 with Altec crossed out and UREI written
in in crayon.
There were certainly speakers with more depth and detail that most of
us recording engineers didn't get to hear until we attended a mastering
session with our hands folded appropriately behind our backs.
The 604 was a studio workhorse with a laudable purpose: When a sweaty
rhythm section blasts through the control room door to hear a playback
of a take they just played at 103dB, you can't give them a playback on
some candy-assed console top monitor at 85dB and expect them to be
excited about the results. 604's had... well they had balls. Kind of a
cross between a big stereo system and a good bar-band PA.
I'm not a beleiver in loud listening for extended periods mind you, but
there's a time and place to crank it up. Overdub and mix sessions can
easily be handled by better sounding nearfields at half the volume.
That's another thing. The 604's DO NOT sound good at low volumes. I
gotta beleive the guy who invented the "Loudness" control on preamps
was listening to a horn loaded duplex at the time.
I agree with a former poster who described them as good sounding on
horns and reeds. (drums and bass too) They make strings sound like
trained bees, however.
I finally sold my last studio pair to a swooning Japanese man about ten
years ago when I tapped into that corner of the world that genuflects
in front of old school, horn loaded Duplexes. He asked me to play some
Beethoven. I said "Suit yourself". When I heard that swarm of bees
buzzing through the crossover area I thought "he'll never buy these old
things". He proceeded to pay me twice what I thought they were worth
and half what he thought they were worth. We both grinned as we loaded
his minivan.