Atma-Sphere in CO?

@cal3713 One other thought is that you mentioned what 6SN7s you had but you didn't mention where you put them.  I have found that any given mixture of 6SN7s will give very different results depending on where the tubes are located in the amps.  In my case with the 3.3's, I have some NOS grey glass RCAs in the two most important positions, (back row, left and right).   I have a Sylvania GTB in the back center position.  This position seems to want a tube with great dynamics, sparkle and air.  When I put a third RCA in that position, everything gets kind of sleepy, almost sirupy.  But the Sylvania that works so nicely in the center position does not play nicely on the outside positions.   In those positions there seems to be a decided lack of smoothness with the Sylvania.  In the 4th position out in front, I've tried new production Sophias and Tung-Sols.  The TS was better than the Sophia, but Ralph has warned against the new Russian TS tubes elsewhere.  The best tube I've tried so far in that position out front is a 1960's RCA GTB.  I've got a pair of GE GTA tubes on the way to try in that position.  In the end, regardless of tube complement, the M-60s are not going to sound like a 300B SET.  I listen to a lot of live orchestral and chamber music, and have a good perspective on what I "should" be hearing.  After 18 months with the M-60s, I would argue the M-60s are more faithful to that live music standard, and in my case I really need the extra juice.  Ralph and others can offer guidance on how to get the best from the M-60's, but no one can tell you what you ought to prefer.   For me, choosing between these amps is a real Sophie's choice.   Its going to take a lot to pry either one from my hands.  There is no wrong choice between these two options.   They both have their own beguiling charm.  Some like blue eyed blonds, some like dark eyed brunets.  I guess that makes me an audio polygamist. 

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.

@brownsfan & @atmasphere great, thanks for the information. I'll do some checking, swap out some of the 6sn7s for some nos ones I've got around, and do some more listening tonight.

How does the dynamics compare from the M60 to the 300B?

On the DIY Forum someone changed the Vcaps to another brand i forgot. The sound was tonally warmer.

@phantom_av Unfortunately it's been too long for me to give any reliable information.

I've transitioned to simple source follower amplification (first watt f4 mono-blocks, only adding current to the signal) and focus on getting the warmth from my preamp (Don Sachs 6sn7).

That combination is remarkably beautiful with great tonality and an amazingly holographic soundstage.