[Phlogiston]

It is easier to have a flat response and little need for equalization if your components are not creating the need.

In a short list of trouble makers are:

Poor crossovers. Even great speakers work poorly with cheap crossovers. First, avoid electrolytic caps. They have their place but it isn't in a crossover. Manufactures save a lot using them. You can upgrade some speakers with a new crossover. Avoid iron core chokes ( practical up to a hennery or two).

Ported speakers. Customers can be impressed with booming base especially if their old system could not muster enough power to make some. BUT a port in an enclosure is going to give a peak at only one freq (based on time and distance). Some manufactures use a flabby bass speaker which produces bass but has poor accuracy/control. See "wave radio" by Blows.

Bi-amp/sub woofer. Freq below 50hz are just not going to come from that 8" base without issues. Consider what your amp must do to produce those low freq too. A 5 watt program might need another 50 or 100 watts for that low base. Use a separate amp and speaker/enclosure. This is a good job for transistorized amps save the tubes for the mains.  An active crossover in your preamp is wonderful. (Ever noticed the preamp out jumpers in good 70's equipment?)(phasing control is also an advantage with many bi-amped systems). This topic could easily be expanded.