The Rotel RB1050 is really not sufficient for one quality subwoofer. Most subwoofer plate amps run at a couple hundred watts, but quality ones run a ton more power through them.
In most things, I think you can get a lot of bang for not a lot of money. Just a few hundred bucks before you start getting into the diminishing returns territory. Subwoofers are mostly not like that. If you get a $2,000 single subwoofer, it produce a LOT more bass than a $500 subwoofer. Even an untrained ear will be able to hear and feel the difference between the two.
So, you will want an amplifier with power to spare. Maybe a crown amp with 500+ watts per channel.
A couple of these would do well, or a single one, then add one in the future. These amps are 2ohm stable, which is good for running a couple of subs on each channel, so 4 subs on a single amplifier. In the future, add a second amp if you want more power to things.
This is JUST an example. I searched for crown amplifiers under $200, and there is a LOT of options, so look up how much power can be delivered by any model you are considering, or ask here for our thoughts on any specific model. But, on a budget, this is DEFINITELY the direction I would go.
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