Who has the best midrange 2A3 or 300B ?

Terrible speaker to pair with a 300B or 2A3 unless you were to find a Push Pull.  You might find a used unit from Opera Consonance that may meet your needs.  

The M100S Plus is a 22w Push Pull using 300Bs and is a darn good sounding amp.  It would be in the right price range for you used. 

Another option would be the Consonance Opera Reference 5.5MKii which is 18w.  Not huge power.  If you can find it use, it will be in the $1100 range.  New, it can be had for $2000 even with the current Chinese Tariff situation.  

You would undeniably be better off with a more powerful amp  A used Rogue Cronus Magnum would be a better choice but if you are dead set on a 300B, those would be your best options.  

2A3s are the ultimate tube in terms of detail IMO.  300Bs have the best mid-range which is lush and addictive.  

Get the Rogue Cronis Magnum; can run in pentode or triode mode.  I Would not run that speakers with triode amp ( 2a3 or 300b).  To answer your questions; no doubt 2a3 is more of an even sounds ( highs, mids and bass).  The 300b tubes will be a better chose in midrange! 

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

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

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

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.

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.