They will get to rated up around 13.8+, but down at 12.6v they will generally fall short of rated with a CLEAN signal. They will do rated with a dirty clipped signal, but not clean .....
I still test everything at 12.6, as that should be the lowest voltage you would get to in a decent car audio set up. lots of people run their systems down into the mid 12v range.... And I sell ALOT of amps on CL, (over 50 so far this year) so I want a true number to give to people ....
I have a test bench with a 500 amp custom PSU that I built for it. I control the voltage through the I2C chip with an Arduino, from 11.9 up to 14.8 .... I am working on getting that up to 1000 amps here soon. Then I can bench amps up to around 12,000 watts RMS ..... I use a Hameg 2 ch Oscope and all my meters are true rms Fluke meters .... I've prob benched around 70+ amps so far this year, I just don't make videos, and do the social media thing with it. I just really like to bench amps, and see what amps really do what, from cheap azz pyramids, to rare 1991 Kenwoods and US Amps, Orion HC stuff, ect ....It's just a hobby I enjoy doing ...... Heck, I've burnt almost a half a dozen subs up in the last 9 months, throwing WAY too much power into them .... it's fun ...lol ...