Atma-Sphere in CO?

@cal3713 I think the amps you bought are MK 3s, so I am going to have to defer to Ralph.  Mine are MK 3.3, so what I know is not relevant to your amps regarding which positions are most impactful.  I can say that at least in my amp, tube rolling the drivers is noticeable.  Some like the old Sophias that were rebranded First Musics, and as I said, I found the 60's RCA GTBs to my liking. (I left the new stock Chinese tubes in the position that controls DC offset, since Ralph told me rolling that tube has little impact.  Not sure which tube in the MK 3 controls that function.)  When you get your amps, please share your thoughts on the amps per se and also anything on your preference in 6SN7s.

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. 

@atmasphere Thanks so much Ralph.  Given the proximity (on my daily commute route) and 45 day trial period, I honestly just bought the amps without even finding out which version they are.  And now they've got the ad down, so I can't check.  My visual memory is that they are are not the long chassis, but I'll find out tomorrow.  Very interested to hear. 

@brownsfan Also, I had been limping around with some old cables on my PREs due to the need for bi-wiring the bass & midrange units and finally got some proper cables in the system... it has greatly improved the situation, so it'll be a good shootout between the Franks and the M-60s.

@atmasphere Thanks so much for the information Ralph.  Given that the amps were local and came with a 45-day return period, I didn't even investigate what version of Mk3s they were.  Definitely the more recent wide chassis version though.  

@brownsfan And very curious to hear them in comparison to the Franks.  Upon getting my PREs out of storage after living away for a year, I've been limping them along with poor cabling due to the separate binding posts for the subs and head units.  Finally got new cables (I built them from the Dueland WE replica wire) and everything is sounding much better with the Franks.  Will be an interesting shootout.  

@atmasphere Also, if I'm reading your post correctly, that means that my RCA 6SN7GT VT-231s won't actually work in the amp.  Right?  I don't think I actually have any GTA or Bs, unfortunately.