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?