can you please help me around how to use my first tube amp?

Hi, I picked up on a whim an older tube amp, first I have ever had that is non automatic biasing and outside of that really no nothing about tube amps

Its a used 10 year old or so Rogue Audio Atlas Magnum power amp with KT 120 power tubes

the pre amp/dac will be RME adi-2 and speakers are Sonus Faber Lumina V's (4ohm)

I was thinking I would also also try it without preamp from a bluesound node

I believe the stock setting is 8ohm on the amp and you need to crack the lid to adjust down to 4

nervous about installing and biasing the tubes

can anybody advise what happens if I run the amp without the tubes being biased correctly?

also, can I run this from the 8ohm default so I dont have to open it up?

finally, how do I know when i need new power tubes and pre tubes?

any other advice will be appreciated