B&W 804s + Atma-Sphere M60 MkIII.2

Per the B&W web site the impedance goes down to 3 ohms. Power wise the M60 sounds like it would be fine. However, the 3 ohm impedance could be a problem. The way around this would be a Zero auto former (anti cables.com) which is placed between the speaker and amp and allows the amp to "see" a higher impedance with easily selectable choices of 2x, 3x and 4x the speakers impedance. So, as an example, if you choose the 3x option the M60 would see a speaker impedance of 9-24 ohms which on paper sounds about perfect but you can easily switch between the options and see which sounds best. It come with a 60 day in home trial so there's lots of time to find out which works best for you. Another benefit, the M60 will put out 80 watts/ch at 16 ohms up from the 60 watts/ch at 8 ohms.

It may sound strange to have an OTL amp only to have but a transformer between the amp and speaker but the Zero autoformer is not like the transformer in a traditional tube amp which has both primary and secondary winding. The Zero only has primary and just enough windings to do the impedance adjustment so it much more transparent than the typical output transformer
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BTW, the Zero is not just for OTL's. Virtually all tube amps will sound better into a higher impedance. For instance, the MC275 mentioned by Lewinski above, IMHO, would benefit from the Zero by not having to play into the impedances below 6 ohms with lower distortion, better bass etc. Some of the biggest users of the Zero's are Magnepan owners with their 4 ohm impedance driving them with tube amps.

Check out anticables.com. Hope this helps.
BTW, my own setup has an Atmasphere S30 driving the 16 ohm Zu Superfly's.

Michel,

I don't use a Zero with the S30 and the 16 ohm Zu's as the Zu's impedance appears ideal for the S30. However there is a body of thought that it might sound better still thru a Zero at 2x looking at an impedance of 32 ohms. I'm happy with my situation as it is.

The Zero is really intended for your kind of situation where you enjoy your existing speakers but the 3 ohm impedance is not ideal for the amps you would like to pair it with. With the Zero you are adding wire and a passive device between the amp and speaker but in situations where it's needed the positives appear to far outweigh the negative.

Regarding changing your speaker, thats a whole other ball game. If you enjoy the B&W's keep them and find an amp that does what your looking for with them.

Lew,

Wasn't trying to steer anybody anywhere. Just trying to answer a technical question of the OP regarding using an OTL with a speaker with a significant impedance dip.
As an aside, it's great that you and your buddy are getting great results with the MC275 driving speakers with low impedance dips. It's a testimony to the excellence of the output transformer and power supply. Even though the output transformer has multiple taps the amp is still "seeing" the impedance dip of the speaker. The question is, would it sound better still with the Zero in place?

Good luck making your choices.