Try the Neuman, but!
I found this on SND Tube Sales on the web RE; the
5751:
"This is the "milspec" version of the 12AX7, but is also slightly different. The 5751 has the same plate resistance as the 12AX7, but the gain factor is only 70 compared to 100 for the 12AX7. The 12AT7(ECC81) also has a gain factor of 70, but since the plate resistance is different, the 5751 is closer to the 12AX7."
Essentially what you have done is replaced the factory
sE ECC81(12AT7) with its gain factor of 70 with the 5751 tube that also has a gain factor of 70 and appears to have some sonic characteristics of the higher gain 12ax7.
You may want to consider going to a 6072A/12ay7 tube that will drop the gain factor another 20 or so from the stock 70. I believe with that specific change you will notice a big difference.

(check this site out, FYI:
https://www.tubeworld.com/6072.htm) A GE 5 star triple mica black plate 6072a would really be a nice gift from Santa!!! Also, when you make that change, try the mic in different pattern settings (you can do this now, in fact). Listen to the mic in full figure 8 then try it in full omni and hear the differences. I just spent some time with my good friend Jeff (
http://www.100yearpicnic.com/) doing the same thing with
my Z5600A mounted in my sE Reflexion Filter and you'd be surprised at the different tonalities you can get.