I find it best to just change the tubes in my boogie every 6 months. I have had tubes that were tested and I was told they still had 90% of thier life left, however I replaced them anyway and it was a world of difference. Tubes are cheap, and if one goes while playing out....it goes and your screwed. Back when I didn't know tone and played thru a Peavey VTM60 (I know, I know....crap!) the amp sounded fine (as fine as a Peavey can), but it blew a fuse in the middle of a set, and blew ever fuse I put in untill I finally replaced the tubes. Didn't notice any change in the tone, though....still sounded like a crappy Peavey.