MC275 in Mono, but which way?

Viper, my MartinLogan CLS's have 86dB sensitivity, which I drive quite successfully with just one MC275. Granted they only go down to 45Hz (gone at 30Hz!). You should have no trouble driving your speakers (whatever they are ;-) with TWO MC 275's in monoblock configuration. My only tip would be to keep the speaker cable under 7 feet to make the most of the tube amps' limited damping factor.

If you listen to loud rock and/or low register (organ) music, you might want to add a sub, but again, that depends on the capability of the main speakers (the identity of which, for some reason, you've so far declined to reveal ;-)

I tend to recommend against biamping if it will be done by running the high and low signals through the speakers' SEPARATED built-in dividing networks. They're designed to function as a whole entity, and when split in two present odd load(s) to the hi and low amps. BI-WIRING is a different situation, and does not present this (loading) problem because essentially the hi and low networks are still strapped together, but at the amp binding posts rather than at the back of the speakers ;-) My position on bi-wiring is that a single pair of really high quality speaker cable will provide the best sound if the speakers have high quality crossovers. If they don't, maybe better speakers are in order ;-)

True bi-amping should be employed when the hi/mid and low drivers have no associated dividing networks in the enclosures; and you are providing that function externally with a high quality (usually electronic) crossover unit (like the excellent Bryston 10B for instance). This provides a wide range of flexibility, but most users will not use them properly or effectively.
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Great amplifier! I wish I had not sold mine.
It is very difficult to bi-amp with different amplifiers and achieve a coherent sound. I have never managed to do it properly, even with matched gain amps. Each amp has its own speed and sound signature.

Option two is a paralell configuration for current-hungry speakers.
From the Mac site:
MONOBLOCK
The two 75-watt channels may be combined into a single channel of 150 watts
into 2, 4 or 8-Ohm loads."
Note that 16 ohms is no longer an option and 2 ohms is available for power-hungry speakers.
Option 2 will probably sound better in the bass than option 1, given that your speakers have low to medium efficiency...however, option 1 (vertically bi-amping) separates the bass signal (heavy current demands on the tubes) from the midrange and treble signal (low voltage, low current, requires extremely low distortion). It will probably sound best regarding midrange cleanliness and soundstaging.

So, I believe we have come full circle: you should try and see what you like best.
Good luck

Nsgarch, my speaker is not very efficient, it's only 85dB efficiency. Bi-amping with different amp is going to be a problem because of different output level. I wonder if you know of any solid state amp that will match well with MC275 when it comes to bi-amping?

Thanks.

If you want to run your MC275's as mono amps (single input IN, and single speaker OUT) with approximately 150 - 180 watts output per amp, there are directions in the Owner's Manual for doing that.

What you are describing above (at least in your Par #1) is using two amps, both still in STEREO configuration, one amp for the mid/hi drivers in each speaker, and the other amp for the woofers in each speaker; This is known as bi-amping, and is a perfectly legitimate way drive your speakers, although most folks who go this route would use a SS amp for the woofers since it provides more control and thus better transient response from the woofers.

Your Par #2 (I think) describes running each amp as a single channel monoblock, one for the left speaker and one for the right speaker. Even without knowing what kind of speakers you'll be driving (a not unimportant piece of information!) I'd be inclined to recommend this as the preferred way to go -- especially since you already have the two MC275's on hand ;-)
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