Depends upon the amp and the conditioner. Conditioners that allow many amplifiers to function without limiting the headroom are very expensive.
This ^^^^^.
Some amps (mainly higher end amps) go to great lengths to optimize the power supplies in the amps. Others can definitely benefit from a good conditioner. It's just not a one size fits all rule of thumb.
I got a chance to buy a used Lightspeed 3200 for $10, so I gave it a try. Wow....really nice with my old tube amps, but can't say what it'd do with other amps. YMMV