Even if I only plan to use it at low volume ?It would most likely still not be a good match, sonically. The M5 has a very uneven impedance curve, most significantly in the mid-range, as seen in Figure 1 of Stereophile's measurements. That will result in an irregular frequency response when used with tube amps that do not have particularly low output impedance/high damping factor.
Also, the combination of a severe phase angle and low impedance occurring at about 4kHz figures to be a particular problem for a low powered amp.
Finally, the M5's are even less efficient than their specs would seem to suggest. Stereophile's measurement was 88db/2.83V/1m, which is only 85db/1W/1m at their 4 ohm nominal impedance (which closely corresponds to their measured impedance in the mid-bass region, where a lot of musical energy occurs).
Regards,
-- Al