Who has the best midrange 2A3 or 300B ?

The M5s are nice speakers, but they wouldn't be my first choice for use with 2A3 or 300B SET amps. Your speakers are 4 ohm impedence and 87db; I would look for something in the 8 or 16 onm range and higher efficiency' say 96db or better.

If you want to use a tube amp with M5s, try the Manley Stingray or Mahis. A Jolida hybris amp would also work great with these speakers.

Even if I only plan to use it at low volume ? This amp will be used with a Leak Throughline tube stereo tuner and a CD player. I will be listening to Jazz CDs, NPR and newsradio.

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

Thanks for the info. I will be looking for a higher powered amp then since I really like this speakers.

I'd look at something based around the EL34 or EL84 tubes. The Manley amps are great with the right tubes.