What is the Altec 604 sound?

It is debatabe as to whether Abby Road used 604s or 605s, or both just changing them out over time.

605s lack the high end of the 604 due to the reduced magnet on the horn.  My brother had a single 605 in a utility cabinet and it was sort of dead sounding even though the horn seemed to work fine.

Altec 605 was a cost reduction version of 604. I has a little bit smaller magnet and sensitivity. But the sensitivity is still very high and weaker magnets give a little more bass (Qts is higher).

Altec 605 monitors were The Beatles monitors in Abby Road studio.

And the 605B?

https://www.technicalaudio.com/pdf/Duplex_Speakers_related_to_Altec_604/DUPLEX.html

What is special about the Altec 604 - The 15 inch driver plays a very wide frequency range up to 1500Hz. The mid bass driver moving mass is very light (compared to JBL and modern Tannoy) to afford to play midrange. Driver quality is on a higher level similar to JBL and better than EV drivers used in Klipsch. Lightweight 15 inch Altec 604 and 515 drivers have better low midrange and upper bass texture compared to JBL drivers, even legendary JBL LE15. JBL drivers have deeper and puncher bass.

The main drawback of 604 is that the 15 inch driver can’t play equally good bass and midrange. The old series: 604, 604b, 604C have a better midrange texture on trade off of bass depth and weight. More modern series have a better puncher bass on price of midrange texture. Old 604 have sectoral--stile horns. More modern models from H, have improved Mantaray horn and phase plug that makes upper mid and high frequencies sound smoother and extended.

Here are my Altec 604E videos:

https://youtu.be/Nlo9qR4-I0c

https://youtu.be/ooFTaeJsdQ8

Model E was in production from 1964 to 1973.

I have Altec 604E from 2005.

I never listened to Altec GPA. I don’t know how do they sound. It can be different from old 604s.

My 604 is very easy to drive and sound is very dynamic without feeling of any compression. The bass is fast and has texture but not deep. In my 9 cubic feet cabinet it doesn’t go deeper than 40hz. High frequencies go up to 10khz, so I use supertweeters.But new GPA should be more extended on high frequencies.

These speakers wonderful for jazz and vocals. They can play rock and classical music too.

They can play in midsize rooms. Even 200 square feet is ok. Because the are coaxial they can play from 8 feet distance.

I have limited exposure to Klipsch speakers, but from what I have read on forums, Klipsch Cornwall can be fatiguing.  I did have a brief listen to Forte IV's and I thought they were ok, but prefer my Altec's,   I typically listed at 65 db average and they sound very much alive and dynamic.

Beautiful system Toddalin, I would come to hear it gladly, but I am in Italy....

Pdreher thanks, The idea would be a 604 or 605 in a cabinet a little less wide than the 620 which is 26'. 
I will look into HiFi Haven.
About the sound, from what you tell me I understand that Altec are perhaps more delicate and less strident than Klipsch (Heritage, Forte, Cornwall)?
At low volume 60db are they still alive? My Avalon Indra (which have 87db sensitivity) at 70/80db are wonderful, but at 60db they fall flat. 

In So Cal? You are welcome to hear my one-off, tri-amped, Super Big Reds using vintage Marantz equipment.  The cabinets are 15.5 cu ft exterior volume.

https://youtu.be/4MWmYYV1bg4

Unlike Altec A5 and A7 series which were designed for theatre use, Altec's paired with 604 drivers are know for their prowess in recording studios... or domestic use in the case of Model 17 and the less common Model 18.  

You should pose your question on the HiFi Haven forum, as I've found the members of that forum to be the most knowledgeable when in comes to horn speakers, especially all things Altec.

I own Altec Model 19's, which are supposedly somewhat similar to A7's but intended for domestic application.   They have a big, dynamic, live, airy sound unlike anything I've heard from previous box speakers I've owned (Dynaudio Confidence C2 & C4... and Harbeth flagship 40 series).  They need a large room to sound good, with considerable distance between speakers and the listener... I sit about 12 feet back.   I've been toying with buying 604's paired with a 620 cabinet down the road, as many Altec aficionado's seem to like them a lot... but some prefer the large A5 and A7 series... but those definitely need a large room and have little to no WAF.