Output Transformer Primary Impedance Affect on Tone

[makinrose]

I was reading TUT4 (I think) that details various transformer set combinations for 50W amps.  With that section was statement about how some builders/players favor the sound of high impedance primaries over lower ones and visa versa.  I've experimented with different primary impedances 5E3 designs but I'm not sure if the difference in impedance was great enough to experience a big difference and I didn't do it with two amp sitting side by side. To my ears the higher impedance transformer seemed to sound little softer and warmer but that could have been my imagination. 

What has been other people's experiences? Also given the same supply voltage what effect should we expect between say a higher and lower impedance primary as far as tone? 

Thanks!