the thing is that a TS in front doesn't really add gain, it just clears up the signal hitting the amp, cutting some low end and adding a different midrange that you can't reproduce by just running the mids on the amp higher. you can dial in more (and tighter) bass on the amp too because of the lowend cut.
having that said, some amps just don't take a boost too well/don't benefit from it....and that's exactly what i'm looking for. i'm so fed up of the live tapdance....with a boost in front you have to push at least two buttons to switch to a clean sound, even more if you start adding fx into the equation and the amp doesn't have some sort of intelligent switching like the marshall jvm. that's pretty much the only reason (aside from money issues
) why i'm not running a rectifier live, it just NEEDS a boost to get into that tight metal territory. 5150s work without one but it's just not the same either....
my JVM on the other hand sounds perfectly fine without a boost....you can run it with a TS and it's a great (but different) sound too, but it doesn't NEED it. probably the reason why it sounds so good in this shootout compared to other amps.
same thing goes for ENGL, they have that low cut already built in so with a TS it just becomes too much. unfortunately most engls sound like ass in my book
in a recording situation i'll almost always prefer it with a TS though.