How do I get a speaker with the bass slam of my el cheapo ear buds...?

in a word, phone is a poor pre-amp.  It can only drive very high efficient earphones, usually cheap.  Because cheap ear bugs don’t have good control, sounds are usually heavily distored.  With strong bass does not mean accurate bass.

you need a good preamp.   But your phone may not have a good impedance matching wih high-end preamps via analog signals anyway.  That’s why people need to buy good sources, like hi-end cd transport or digital network player to get a high quality sound.  

But if you would still want to use phone and hope to get a good sound for your stereo system., you can try using a good headphone amp as pre-amp, then connect to your stereo.  Or you can go “digital” transaction method.  i.e.  using a portable DAC/headphone amp.  like Audioquest dragon fly or Fiio’s DAC/headphone amp.  Also make sure your stereo system can deliver a good bass slam first.

The portable DAC/headphone amp can also be used for car stereo purpose.  Good luck.

What speakers in your stereo system?
i think you have never heard a good stereo system if you think bass performance from a earbud is impressive. Yes the earbud can go low and loud but it is nothing like true bass from an actual woofer of substantial size in an appropriately tuned cabinet and volume 

Headphones can have bass slam, but it doesn't compare to the sound quality or sensory impact of sound waves interacting with your entire body.

My high end headphones and headphone amp don't come close to producing the slam and impact of my Legacy Audio Focus 20/20 speakers (or any of the smaller systems I have set up in my house). 

There's no comparison between a driver that would fit in an earbud and a large woofer(s) and/or subwoofers.