Making speakers sound tonally similar with an equalizer

In an anechoic space you can equalize and correct the phase of speakers by using FIR filters and make them sound more similar on axis than I think most people would admit, at least up to the volume where the speaker starts to distort audibly. In real listening environments it’s not possible to make speakers with different cabinet shapes and arrangements of drivers to sound indistinguishable because they radiate sound into the room differently and so the combined direct and reflected sound effect is quite noticeably different no matter how you equalize or phase correct. I would bet though that if you took two different reasonably well designed speakers with the same baffle shape, same driver sizes and driver placements on the baffles, let’s say a 5" woofer and 1" dome tweeter on 16" x 7" baffle, they could be equalized and phase corrected to sound similar enough under blind testing that most typical listeners and even a lot of seasoned audiophiles would have a hard time distinguishing them. Any differences that could be heard would most likely be from differences in the specific dispersion characteristics of the different drivers chosen.  

Short answer, no.

You can’t easily improve dynamics, dispersion or bandwidth via equalisation. Not without introducing additional distortion artefacts.

However you could make their frequency responses sound a lot closer.

In fact quite a few people already do this with headphones.

They simply dial in their saved preferred equalisation via the method of choice for every headphone they use. I tend to use some equalisation for every headphone I use. It’s not always easy as what can sound like an improvement with certain music/recordings can sound like distortion with other types.

Dynamic range matters.  A lot of highly regarded speakers have a lot of compression and distortion which is impossible to replicate with simple EQ.

Then the issue is that there is no single universally accepted standard for measuring the frequency response of a speaker in a room.  Consider a planar or horn speaker, vs. traditional dynamic. The integration of the speaker's signal with the room over time varies a great deal here.

I will not agree to making one speaker sound like another similarly designed speaker by way of EQ alone. Even if that were the challenge, specific drivers can be different as according to the magnets used, surround material, cone material and more. How would you replicate 'speed' or even extension if a driver simply cannot comply? In some cases, it would more likely become attainable if certain speaker parameters were derivable.

 Having said this, I used a program that tried to do just what you asked. I'm sorry, but I forget the name. Point was that not only EQ was used, but variety of other changes were produced with this 'program'. The trouble was, even though the different programming made my speakers/headphones sound different, I could not say as to how close they came to the replication of a chosen speaker or concert hall, etc.

 It was fascinating to be sure, but because I am not very good when it comes to technology, frustration won over curiosity.

If you are asking the question of high fidelity consumer equipment… no. Equalization always has negative impacts on sound… so while you might make a few gross changes… you are going to lose fidelity by putting a bunch of pots or digitally manipulating the signal. For high fidelity the only serious game in town is get the signal right, pass it as transparently as possible through the amplification chain to the transducers.
 

In a high end research and development facility with many many hundreds of thousands of dollars worth of equipment,  sure.