2.1 System Upgrade Advice

First post, hopefully this isn't redundant!

Current setup is my first that I put together a few years back:

Sony STRDH190, Sony SSCS5s, Orbit Turntable

My initial instinct is to add a sub (which one?) but was curious if anyone had another suggestion of a next step upgrade. Simply put, if this was your setup, and you had $300 to upgrade it, what would you do?

Thanks!