so i want to make the el-34 to 6l6 swap but i want to make perfecly sure i'm good to do so with my amp.

i have a laney gh-50L
i want just a little bit more gain out of my amp
i was told 6l6 tubes get a bit more gain.

the amp says that it is 5881 compatible which is the same as 6l6 i know.
so i'm pretty certain i'm alright there

what are the best 6l6 tubes for say: a metal tone?
i notice lots of variations of the 6l6 wherever i look for them.

also i noticed that alot of ppl swap out 12ax7 with 12at7 for less gain

can you do the same with the ef86? or cc83? pardon me if my names are wrong.
essentially are all preamp tubes cross compatible?
you wont really get much more gain swapping tubes i think, if you get higher gain pre amp tubes you will get more pre amp gain but you would be better off getting that from a good pedal, just buy a high end pedal its easier and sounds all good.
You may want to buy a set of rated tubes like Groove Tubes.

Groove Tubes let you buy tubes rated 1 - 10. I believe 10 is the cleanest and 1 is the dirtiest.

I had to put a 3 into one of my amps and it was very dirty. I can't imagine who would want a 1.

Generally, your preamp tubes are where your gain stage is. An EL-34 is essentially the same tube as a 6L6 but you may have to re-bias if you do this - unless you have a self biasing amp which I am guessing the Sound City is not.