Upgrade my amp again

the Musical Fidelity M3i is 76 wpc, but that is 76 into 8 ohms and 137 into 4 ohms. My assumption is that this amp would be powerful enough (because of the 137 number.) Is this correct?

Margot, the MF M3i would definitely be much better than the Marantz pm5004, and it may be powerful enough with a 3db increase at 8 ohms and approximately a 4.5db increase at 4 ohms, however the required power is determined by a relationship between the peaks of listening volume, listening distance, and speaker sensitivity. As far as the 8 or 4 ohm rating, when a manufacturer recommends a power rating for a speaker, this is generally the 8 ohm rating. The 4 ohm rating for an amp helps determine how substantial the power supply is, and its ability to deliver the power into a demanding load the speaker may present as music is played.

Take a look at the owners’ manual for the PSB T6 here; section V- D. (Recommended Power),

http://www.psbspeakers.com/content/110616100531-PSB_OM-200ImageImagine_English_Manual.pdf

The manual even recommends a minimum of 50wpc for room filling volume. The MF M3i may be adequate for your needs, but the only way to know is try it, or get a SPL meter as Bob stated, measure maximum peaks in various music, and do the calculations. For more power in the price range you stated, the Creek Evolution 5350 and the NAD C 375BEE are good choices. Also, with more power, The Peachtree Nova125 would offer two choices in the character of sound by switching its’ tube buffer in and out. It also has digital inputs, but only one analog input.

Phaelon has brought up a very good point about how the speakers are coupled to the floor. Again, look at the owners’ manual in section I-B-2. (Molded Feet and Stabilizers). Are you using either the spikes for carpet, or rubber feet for a hardwood floor?

I agree, interesting discussion with different opinions, so here’s mine.

No doubt a different speaker could be better suited for the room, however I think you definitely need more power on the current speaker.

they sound great with jazz or acoustic. With rock, they can sound a bit muddy

Since you said they sound great on jazz or acoustic implies the speakers are working OK in the room. Rock music is generally played somewhat louder than the others stated due to the tendency of “Let’s rock, crank it up!”, therefore requiring more power from the amp. The Marantz pm5004 is a good value, but with its rated power of 35w @ 8 ohms, and 45w @ 4 ohms, should be used on a less demanding speaker. Reviews and measurements on the PSB T6 indicate it should be used with a substantial 4 ohm rated amp. Also, considering the speakers’ measured sensitivity, average listening distance, and moderately loud listening volume for rock, it’s easy for me to see that the power requirement could exceed the Marantz pm5004.

And is the general issue about right amp for a speaker about the ratio (ie double the watts at 4 ohms) or is there an absolute number? Ie: ideally you would like 80 watts for a 4 ohm speaker?

Basic speaker/amp matching is generally done using the speakers’ sensitivity. Take a look here,

http://www.diyaudio.com/wiki/Loudness_and_Speaker_Sensitivity

This is all based on a 1 meter distance, and since you are probably further away, the reduction of perceived loudness at greater distances must be considered. The approximate reduction will be, 2M – 6db, 3M – 9.5db, 4M – 12db, 5M – 14db, 6M – 15.5db. Obviously, the greatest effect is the first 4 meters.

A 4 ohm speaker is more demanding than an 8 ohm. The amp should be rated for 4 ohm, or even better, rated for 2 ohm. It is definitely a big plus for the amp to double its power into a 4 ohm load, but not absolutely necessary, although you would want to see a substantial increase, much more than the 30% increase of the pm5004.

In speaker/amp matching, it is always better to err on the side of too much power than too little. With too little power, the desired sound cannot be achieved, and there is a greater risk of damage to the speaker, amp, or both. Since a double of power is only a 3db increase, I believe I would be looking at amps in the 100w range and above.

As Captain Kirk said, “Scotty, we need more power!”
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