What do I need?

@tochsii You can power a home theater with 2 channels just fine. You will not get any surround functions, but you can't miss what you never had. 

If you’re gonna now have additional speakers for HT the cheapest/easiest thing would be to get an A/V receiver with front L/R pre outs that you can feed to your stereo preamp and power the other speakers.  If your stereo pre doesn’t have a HT bypass this can still work very easily using any unused line level input on the preamp, which I can elaborate on if needed.  

Get a good sub and run it either off pre outs or speaker level, whichever you system dictates and be done. 
 

I have been running 2.1 so to speak for 20 years in various systems and it has always sounded great. 

Look into buying a used Classe SSP-800 AV processor. It has an LFE out for a sub and you're good to go. Does your preamp have Home Cinema Pass through? 

I have a denon avr with 15 channel s these days they have atmosphere witch puts front high middle high and rear high .this is so when things in the movies rly overhead they go from front to back..some music is now in atmos.ir your preamp has home theater pass through it allows the two front channels to be driven by your current amp.home theater can go from mild like stuff at best buy or separate home theater with separate multichannel processors with 30 separate monoamps for 15 channels.this is why mcintosh made the mi 502 class d amp for home theater.it does sound good in stero as well. You might have to read some home theater articles..personally I like my denon avr with 15 channels.its hard to go back to the theater.enjoy your search.and the music.