guitar amplifier going bad ..... why???

Should i have replaced these transistors in pairs? I pulled the transistor that just went bad, checked all diodes ect.. around the output stage and turned it back on. I made sure that the lone transistor was not heating up too much and left in on for a min or so, and no damagae was done. Seems to be working now with just one, so i am gonna put it back together and see if it works.

ME AGAIN. Same amp

Okay, i am really lost now. I used my amp since i last replied to this thread using only the one negative rail power transistor (instead of 2). WEll last week i ordered 2 TIP 147 replacements and put them in. Nothing shorted, but i can hear anything coming out of the amp now. If i turn the volume up all the way i can barely hear the guitar. The wierd thing is when i power it off, it gets really loud (guitar, not noise) as soon as i hit the switch, then obiviously powers off. What do i test now??? what could be wrong. Thanks for any help guys.