Without any form of bass management - as I've said, any DAC with balanced and unbalanced outputs and volume control can be connected to a sub.ok, but who makes a DAC with a subwoofer output, or better yet DSP, for <$300? I don't know of any.
As for a DSP the only one I can think of is the 2x4hd.
This particular device is $469, so are you looking for this level of DAC performance, inputs and DSP features for under $300? At the moment that isn't possible, but an unbalanced Flex is only $30 more than a DO200 and gets pretty close.
For under $300 you'll need to compromise somewhere.