ATC has a subwoofer. I have heard it and it is very good from a cross-over perspective.
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I'd suggest an SVS PB13Ultra with all the ports sealed might be your best bet (based on what I have seen of its performance measurements - so be warned I haven't experimented with this combo) If you can find one used then it will be close to your target price. You need a tight fast sub (why it must be sealed) with high output without compression (why it must be big) because your small monitors have the largest drive motor of any 6 inch woofer in the world (according to Sonicflare) and therefore it will simply be very hard for anything smaller to keep up (ultra-LF bass with similar dynamics to what you are already getting above 80 Hz while maintaining low distortion is physically very hard to do no matter the manufacturer)
The ATC C4 sub would probably be ideal but it is way more $$ than the SVS.