Hi there,hard to get much info in the title.Basically im picking up a used lonestar special online.Cant try before i buy so i was asking as many questions as possible.I asked about the tubes and he said he did change 1 power tube.But arent they supposed to be changed in pairs so they match?I do know mesa color codes matched pairs(green,yellow etc) so for example if a yellow power tube was replaced with another yellow power tube, would it be ok to change 1 at a time?Cheers