Mumbodave

Hey Myles, One of the poweramp tubes in my amp has blown! I own a H&K Duotone Head (100W) which currently houses 4 TAD EL34B-STR tubes. How many of the EL34s do I need to replace (the amp was made in 2003)? Does it matter which manufacturer I buy the tubes from (I was thinking of getting some Rubies)? Do they need to be EL34B-STRs for me to not need to bias them? What does biasing mean (and can I do it from home?!)?! Thankyou for your time, and apologies for the myriad of questions! Dave