Atma-Sphere in CO?

I'd love to be able to just leave the stereo on all day without concern for tube life.

If the amps are on a moderate load (and most Coincidents are) then the tube life is really really long. I leave my amps on all day and most of the night without worries, although I do put them in standby if I step out for dinner or the like.

If its an older amp updated to Mk3, the tube layout might be different.
So, in the latest iteration, the tubes in the rear beneath the meter on either side are the most important to the sound. The one in the middle is next most important.
In the very early Mk3.0 (long chassis) the most important tube is the one in the front, followed by the one right behind it.
In older amps updated to Mk3.0 (and up to Mk3.2) that have the more rectangular power supply cover, the front left tube is most important, followed by the front right.  If updated to Mk3.3, both front tubes have equal importance.
The driver tube sits by itself on the later chassis (with the more curvaceous power supply cover). It should have either a -GTA or -GTB suffix. If the long chassis version, this is the rearmost 6SN7. In the older rectangular style, this is the rear right hand 6SN7. While this tube has less effect than the others, it is still audible.

One thing to keep in mind- you may run into intermittent operation when trying different tubes. We use the Chinese 6SN7s, which have a longer pin on the base of the tube that is also slightly larger diameter. So the socket pins might be spread a bit and have a looser contact with NOS tubes which often have shorter pins. The sockets are easily tightened without taking the amp apart. Contact me if you run into something like this and I'll walk you through the process- it only takes a few minutes. 

The GTs will work in all but the front position. Just use the Chinese 6SN7 in that location and you'll be fine.

The fundamental difference I suspect you'll hear is that the Franks have a more lush sound while the M-60 is more neutral. The difference is that the distortion signature of the Franks is based on the 2nd harmonic while the M-60's product (like all fully differential amps) is based on the 3rd. The ear treats both the same, but amps with a 3rd harmonic have overall a lot less of *any* harmonic, starting with the 3rd which is about 1/10th or less of what you see in an SET. So while it is still smooth and relaxed like an SET, its just got less of its own 'tonality'.

I'd be interested to hear what you think.

Just a FWIW- we've had terrible luck with the MIT and Transparent cables; we've never gotten them to play nice with our gear. No idea why, but you might want to run that comparison again if you can borrow a different set of interconnects.

Also- the amps are very transparent, but if they've been sitting a while, I wouldn't take them seriously until they have about 20-30 hours on them. Further, they need about an hour to really sound right.

@cal3713
It sounds like these amps were used- any idea what version they are? The current version is Mk3.3 (older versions can be updated with full warranty reactivation).

If the 6SN7s get weak, the amp can sound a little sleepy or dead. Its worth it to make sure they are all good!

If it had rounded corners on the power supply cover its a Mk3 at least.
Any idea of the condition of the tubes? The main reason we've found that people let go of our amps is that they don't realize how far downhill the tubes have gotten. The second reason is that they buy a new speaker to which the amp is not compatible. In your case, that is no worries as your speaker (or its predecessor) was designed with M-60s that Israel used to own.

while we can test the tubes, that may not really give you the information you are looking for, we can test to see if it is working, and get some idea of the condition but this won't tell how it sounds or how much use they have had. In addition, this would also take quite a bit of time to test that number of tubes.

What you want them to do is test the 6SN7s. If they are good, that helps a lot! They should not be borderline. Don't have them test the power tubes, instead you can do that quickly by comparing them to the photos at this link:http://www.atma-sphere.com/Resources/6AS7G_Visual_Inspection.php
The reason is that if the tube is failing, quite often it knocks out the link or links in the photos. So a tester really isn't needed- the amp has a meter, if you are concerned about strength, and it can be used to get a relative merit reading of any power tube.