Welcome to the forum!
In terms of buying an amp or getting a new sub, what sub do you currently own? Were you happy with the sub's performance when it was in your old system? It sounds like it was part of speaker system or home-theater-in-a-box set up, and if so, you can just let us know what system you have.
As for powering your current sub, you could use the receiver that you had (if you haven't given it away, yet) or any other older receiver that has RCA inputs. That's likely a temporary fix, but you can use it to try out the sub with your new set up. You would run the subwoofer output from your new receiver to an RCA y-splitter (if required), then to one of the audio inputs on the back of the old receiver, then connect the old sub to the old receiver using the speaker-level connections like you did before. The volume control on the old receiver would act just like the gain control on a subwoofer amp. I did something
similar a few years back. Don't be surprised if you need to turn the volume control on the old receiver up quite high.
