Your post really makes no sense. When you specify a ".1" channel you
DO say you want to use a subwoofer for stereo, so what's the beef with using one now?
BSA's first paragraph is a bit harsh, even by my sarcastic, sadistic standards, but what follows bears repeating. You should strongly consider this advice.
If you want to play louder - get bigger amp, if you want more bass - get a sub and if you don't like the sound at all - replace the speakers
simply changing receivers would do next nothing
A bigger amp won't add bass. to grant wish, the genie will have to give you either a sub or new speakers.
There is no ".1" channel in two channel music. It's just summed mono from both channels with a low pass filter and a powered sub's internal crossover can easily handle that task. You can connect virtually any powered sub to virtually any stereo amp.
But, yes you should state a budget