Need help with sub selection

Downsizing to monitors (monitor audio silver 3's) for a while,
with a buget of $500 (used) any recommendations on a small sub
no larger than 10" driver.

Thanks in advance
The Onix Rocket UFW-10 is hard to beat in a small to medium sized room. It won't extend into the 20s but does very well into the low 30s. But most importantly it is tight, articulate bass, not boomy HT type bass. It's s little over your budget at $600.00, but well worth if it you can stretch.

You could take a look at one of the Q series Rel subs. I don't have any extended experience with one (only at an in-store demo). Sounds clean and a fair amount of adjustments. An older Vandersteen 2w (non q) would be fairly close to your budget.
Vandersteen 2W utilizes a trio of 8" woofers and a self contained 300 watt amplifier. Used prices run under $500. IMO, the 2W is a great value.
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Given your budget, you may want to consider the Outlaw Audio subwoofers, which were designed for them by Hsu. They have gotten good reviews, and offer a lot of performance for the money. Click on the following link to Outlaw to learn more: