tooblue - I seriously need to update my virtual system. Yes, the mb125s eventually replaced the st150, but there were a few other amps in between. Same for speakers. I replaced the wilsons with quads, but eventually wanted to go back to dynamic speakers, ultimately "settling" on a pair of vandersteen quads. I'd venture that most of the difference between the mb125 and the st150 is due to el34 vs 6550. BUT ... I HAD to replace my st150, which failed catastrophically in a way that was simply not cost effective to repair - I can't say I did it as a direct upgrade, and there are too many other variables for me to make any reliable comparison.
Thanks.