$200-$300 will not buy you a very good sounding subwoofer. Keep your eyes open for a used Martin Logan Dynamo 1000W, (hopefully around $400 used). They are excellent for music, very un-colored and "quick" sounding. Also, a used REL T-7 series for around $500.
Subwoofer suggestions for budget system?
My current setup is a pair of Magnepan 1.6 speakers, a pair of Schiit Vidars, Schiit Sol turntable w/Goldring E3 cartridge (I have a VAS-rebuilt Denon DL-103 with Ebony shell and microridge stylus when I can get someone to come set it up for me), Mani 2 phono stage, Asgard 3 headphone amp acting as preamp, Schiit SYS as source selector, Modi 3+ connected to Volumio for streaming digital and an Orchard Audio RCA-to-XLR converter to drive the Vidars in monoblock mode.
I'd like to add a subwoofer to the mix that can keep bass out of the Maggies. I'd, of course, prefer a dipole sub to go with the dipole speakers, but not terribly picky ATM. My budget is around $200 to $300 for now. I have been very tempted to build a pair of my own subs, but that's not in my budget right now.
Suggestions?