Marshall retube Q's??

Hi guys,
I have a Marshall JCM900 SL-X 50watt head(all tube no diode 900 head).Anyhow its a few years old now and getting near retube time,I'm surprised they lasted this long.Anyway I am getting a matched pair of Electro Harmonix EL34s and (4)12ax7 Electro Harmonix.The 12ax7s rated high marks for microphonics and noise and 4 outta 5 for tone.And the EL34s are spossa sound similar too Mullards.
Anyway is anyone familiar with the SL-X(super lead Xtended)heads and do they need to be biased when changing tubes??
How much is a rebias??
The amp is a 98or1999 I beleive.

There is no need to re-bias for the 12AX7 preamp tubes, but when you change the power tubes (EL34 in your case) in any amp, the bias must be adjusted. It's a simple thing for a qualified tech - 'shouldn't cost much at all.