few questions about my amp, pls come in

[kskr]

hi ya all!

It's been more than a year after building my first amp ,
and still have been trying to improve it.
Here I've a few questions, hope you experts there would give me some hints.

My poweramp schematic is a SE, tetrode mode,
with 1/2 12at7 as driver stage + single 6v6GT as output stage (with 3k CFB).
Speaker using very low sensitivity ProAc speaker (86 db) .

1) I saw a lot schematics that has paralleled driver stage,
how does it improve the sound besides lower distortion?

2) I notice and checked my amp's controllability is very bad,
seems to mess up with audio output at around 120hz area
(filtering problem? will it be better with tube rectifier?)
It's strange because I almost can't hear any hum with my ear close to the speaker.
Any suggestion on better handling around 120hz? will paralleled driver stage help?

3) Could anyone give me a link of the loadline of 6v6? Because I can't seem to find one.

4) Will 6sn7 as driver perform better than 12at7? because i heard somewhere said
if gain is not critical, try to use the lower one. Because of lower internal resistance?

Many thanks!!