People complain about lack of bass, not enough mid range. Solution?

You are not defining the Quality of the replacement speaker ,is it a 2 way to a 3 way speaker if so the quality ,and size difference . Your room can excite different bass frequencies ,and what’s called suck out in certain rooms . If if ported maybe ports are different. Having room treatments helps for sure, rugs in front of speakers .you did not give ample information to have any idea of the brand any differences .

just changing tweeters , and cabinet driver types ,and theSynergy between the amp and speakers ,power doesnot guarantee synergy . .having. Been in audio sales 

over 20 years in the past there are a lot of factors .if you mention your part, and present speaker, amp current amplifier your using , efficiency , and what’s not the same or likes, dislikes between the 2 then you can create a picture ,and room , is there anything different there ? Glass can make a big difference reflections especially if a different speaker ,like going from a monitor to a floor stander .

Ok, I get it now. No room acoustic treatments needed and bi-amping is the key. I'm doing it all wrong.

Try optimising speaker placement. Then, try some room treatment (diffusers, absorbers, etc) - As other have noted above, you might find the bass magically appears!

Following the above you may wish to bi-amp -- or not, as it were.

OTOH, if you liek the idea of two bi-amping don't deprive yourself; but you'll still benefit doing the above

Geting the bass "right" should improve midrange detail and linearity, not detract from it.

As a starting point rule of thumb, you want the distance between the tweeters to be about 83% of the distance from your ear to either tweeter.  (Unless you have one of those rare speakers designed to be in a corner or against the wall.)

Most speakers need to be at least 2 feet from the wall, up to about 5 feet, unless you've got a huge room.  So maintaining the 83% ratio, experiment with moving the speakers in that 2-5 feet from the wall range.

Unless something is damaged or mis-wired in your speakers, speaker placement is 90% of the ball game in getting frequency response right.