An FR speaker SHOULD only put out what you put into it, but I don't believe there is a PERFECTLY FLAT speaker anywhere. I've never seen a frequency response chart for a speaker that was a flat line.
But yes, there is.
A frequency response chart does not need to be perfectly flat. What matters is that there is no tilted frequency response, that there is no larger regions differ completely from the zero line, and no smaller regions differ more than 1 or 2 dB.
Many high quality monitor speakers are very flat. I took the Neumann 310 as a reference. Have a look at its frequency response and deviation. World class!
I compared the FR sound of the Kabinet to the Neumann and applied tweaks. I spent another week with just listening A/B, but could not find any more frequency regions to correct. The last tweak was somewhere in the mid region and it was 1 dB.