Atma-sphere M60 II.3-Any good for 4 ohm speaker?

As you suggest, the Rendition Audio Monitor 60 is designed by Ralph Karsten; it's styling goes back to the earlier Atma-Sphere M-60 styling that Ralph always liked. The circuit is very close to the current M-60 MkII.3 but in a different chassis.
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711Smilin, what you see in the power output specifications of the M-60 into different impedance loads (power goes down as impedance goes down and up as impedance goes up) is characteristic of OTLs. (As contrasted to solid state where the reverse happens.) It's one of the reason OTLs traditionally do a good job with electrostatic speakers which can have dramatically rising impedances in the top end.

I don't doubt Trelja's description of his experience with 4-ohm Frieds. But the "conventional wisdom" is not just "urban myth", there's a lot of solid reality to it. Trying to drive 4-ohm speakers with M-60s will be an iffy proposition, and not just due to the reduced power. There will be increased challenges around controlling the woofers effectively due to the impedance mis-match of the M-60 to the 4-ohm speaker load. With certain specific drivers, the matching may work; apparently it does so with Trelja's Freids, but I'd be very hesitant drawing broad conclusions from a single specific example (which I think is an exception). You may get a similar result with your Reimer Tetons, but you may not.

The greater number of output tubes in the MA-1 make it a much more suitable match to a benign 4-ohm impedance curve than the M-60. The increased number of tubes in the MA-1 lowers the output impedance of the amplifier by half (compared to the M-60) and makes for better power transfer into the lower impedance load and better control over the drivers. It's not just a function of the MA-1 having higher rated power output: it's also a function of better controlling the low impedance speaker's driver.

The Zero autoformer can help. But it's not a panacea, and while there are reports from many folks who have been very pleased with the results obtained with the Zeros in their systems, there are also reports from people who not positive about the reulting sound quality.

The best way to get advice about the compatibility of the M-60s with your Reimer Tetons is to call the factory and ask Bill or Ralph: 651-690-2246 If they've had any experience with your speakers, they'll give you a straight answer about compatibility.
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Fatparrot,

Thanks for clarifying that "Ralph is back!" It is good to have him available at A-S once again to talk with.

As to the 12SX7 mod, I haven't heard about this creating any problems. The main considerations for Ralph when I asked about this mod are: (1) increasing cost of the 12SX7 on the vintage tube market; (2) significantly decreasing availability and no "new" production; (3) much better availability of the 6SN7, including great sounding vintage stock (although the vintage is also getting more costly). In short, he's not a fan of this mod and doesn't recommend it for these reasons even though many people report liking the sonic change.
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Jwin, I've not heard this combination, but they might work depending on the minimum impedance, how smooth the impedance curve is, and the characteristics of the bass driver. Adding a "Zero" autoformer might be useful. The best answer would come from Atma-Sphere. If you call or email Ralph, he can tell you if he has customers using this combination successfully.

What I can say is that the Atma-Sphere MA-1 Mk.II is a stunning amplifier in its neutrality, transparency, speed and (with complementary speakers) tremendous bass control and impact. This is coupled with exceptionally truthful rendering of harmonic overtones and an overall musicality that is instantly addictive. Good Luck!
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