Here's an example.I don't know of a subwoofer that has independent L/R XLR inputs. I know the monoprice subwoofers I reviewed (below) have a single XLR input. You may have to go that route and build your own XLR summer.
This guy has some 2-to-1 summers for sale - if you follow his links - for $43. http://silentsky.net/wordpress/archives/863
I am not aware of any in this price range, except if you find one B-stock; like 'Dynaudio 9S' [available in Europe].[...] in the $500-700ish range [...]
The Kali sub seems to have them. https://www.kaliaudio.com/watts
Beauty isn't too important to me. It's going under a desk. Performance is much more importantI don't know this segment either, but on specs I can't imagine it getting much better than that in this price range.
But on the speaker beauty scale of PA speakers to whatever your pick for the most beautiful speakers in the world thread is, that's way down towards PA speakers. Maybe not a big deal if the OP is putting it behind or under desk or anywhere else mostly unseen.
I added a columns "Analog XLR inputs" and 'High Pass Filter' to my 'subwoofer comparison' spreadsheet, so you can filter for it.Do these studio subs like the JBL LSR310S also HPF the outputs to the monitors?