Which brand of 45 tubes do you use?

clio09,  Thanks for your detailed description of the subject OTLs.

The OTL amp(s) will be used to power just our towers which play down to 120 Hz. The speakers have powered woofers. Do you think the "better low frequencies" of the M60 fall below that range? I personally don't like overblown bass. Good definition matters. If the recording has lots of bass, so be it. If not, a system shouldn't add or overemphasize what's there.

My understanding is that the Music Reference OTL-1 is no longer available and that few were made.

Even if the cut off is 120 Hz I would still opt for the M60. I crossed my ESL57 at 100 Hz and while the S30 was fine, the M60 just gave me more.

A revival of the OTL-1 and a new Music Reference OTL are on the drawing board. Progress is somewhat at a standstill right now though for obvious reasons.

i prefer EML 45 solid plate to National Union NOS ST 45 on my Jeff Korneff amp. The NOS is very nice but EML is even better. 

KR makes a 45 that they ship with their P-135 Preamp. Most KR tubes are designed so that although output is consistent with the spec for the tube, the plate dissipation is higher than spec allowing you to drive the tube harder to get more out of it without sacrificing tube life.

I have retail pricing on this tube at $850 a matched pair but don’t have the spec’s or data sheet. I reached out to Eunice and Marek to get it. I ought to have the data sheets on Monday. They are usually super responsive.

An interesting amp to try with your speakers might be the Art Audio PX25.  It is 5w amp and obviously uses the PX25 tube.  The PX25 is between a 300B and 2A3 in terms of sound profile where it has a midrange that is lush but not as lush as a 300B and it offers more detail than a 300B but not quite what you would get from a 2A3.  It sounds closer to a 300B than a 2A3.  Pricing depends on a few variable but it is an interesting amp.  

I think Art Audio may have been mentioned to you by a mutual friend. PM Me if you want more info on the tube or amps.

I have 4x EML mesh plate bottle 45 tubes running in my triode labs parallel integrated.  Best sound I’ve heard.  Yes the EML’s are pretty expensive.  Nothing comes close to this amp in accurate musical reproduction.  
Omega super alnico monitors running very high efficiency helps create the immediacy.  A small sub was a perfect addition.  
Not sure if the mesh plate Sophia ´s would sound better.  I haven’t heard them.