Can my MC275 power speakers with 91db sensitivity

First, as a point of information, keep in mind that since 2.83 volts into 4 ohms corresponds to 2 watts, the speakers will just put out 88 db for an input of 1 watt, at 1 meter. That is particularly significant in the case of a tube amp such as the MC275, since tube amps generally cannot supply much more power into 4 ohms than into 8 ohms. (For 8 ohm speakers this is not an issue, because 2.83 volts into 8 ohms corresponds to 1 watt).

It can be calculated that 75 watts into the two speakers will result in an SPL of around 100 db at a distance of about 10 feet, neglecting room reflections and thermal compression that may occur in the speakers at high volumes. That is good enough for most listeners with most recordings.

However, if you listen to recordings having particularly wide dynamic range (the DIFFERENCE in volume between the softest and the loudest notes), such as some well engineered, minimally compressed classical symphonic recordings, you might find the amp running out of steam on occasional dynamic peaks. If your listening is mainly to recordings that are highly compressed and/or have little dynamic range, such as most rock recordings, and you don't listen at unusually high volume levels or at an unusually great distance, you won't have a problem.

Perhaps a more important concern, though, is what the amplifier will sound like when driving the speakers. If the speakers are the same version of the Cremona that John Atkinson measured here, or if they are a different version having similar impedance characteristics, I would have some concern that their impedance characteristics in some parts of the bass region might be a bit difficult for this amp to deal with gracefully. Note that the impedance magnitude is only around 3 ohms from 80 to 200 Hz (where a lot of power is often required), and as JA notes there is a very challenging phase angle just below that region. And, not surprisingly given those characteristics, I see that JA mentions that the Cremonas "seemed more sensitive than normal to amplifier choice."

Hopefully someone who has tried this particular combination will comment further. Although, again, the dynamic range of the kinds of recordings that are listened to by the particular individual, and the listening distance, are important variables.

Regards,
-- Al

Wow. Thank you Al for the informative response. (I feel like I might be given a test on your post at the end of the semester). I primarily listen to blue note jazz records, female vocal records (ella, diana krall, billie holiday, etc), and classical. All on vinyl. Perhaps I should look for a less sensitive speaker? Or, could I solve this issue by adding a second amp? Please note that I could not add a second amp for long time.

I see that the amp is rated to deliver 150 watts into 2, 4, or 8 ohms in paralleled mono mode. So adding a second amp might very well be beneficial, but given the expense and the wait that would be involved, as well as some remaining uncertainty, personally I would look for a different speaker that has more benign impedance characteristics. Unless an audition of the particular combination turns out to be possible, or persuasive anecdotal indications about the particular combination turn up.

In any event, good luck as you proceed. Regards,
-- Al

Could you recommend a such a speaker?

My recommendation would be to try Tannoy, something like the Kensington, which, if you find on the used market, should be similar in price to used Cremonas, or maybe even less.

Although my Tannoy HPD 315's (12" Dual Concentric) are custom built, their drive characteristics probably aren't too different from the Kensingtons.

The point being, I am able use two quite different amps to successfully drive them in a fairly large room (16' X 34' W/cathedral ceiling). A very powerful VAC PA 35.35 push-pull EL 34 amp @35Wpc, and a SET 300B amp at 9Wpc.

Try to get out and audition some Tannoy speakers, and see what you think.