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

The solution is in the cup of coffee!

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.