Subwoofer suggestions for budget system?

The Dayton SUB1200 or 1500 are within budget and can be adjusted to be quite effective and tolerable to enhance your low bass output.  Set the low pass crossover frequency at its lowest setting, and keep the gain down so it's barely noticeable except for passages that contain deep bass.  Experiment with placement and phase to suit.

My subwoofer suggestion for a budget system is: none. Don’t do it. Especially not for $2-300. In that budget range you can buy Townshend Pods, Cable Cradles, Purple Fuse, HFT, or any of a number of interconnects, power cords or even speaker cables that will have a bigger and entirely much more positive impact. Or you can get one seriously compromised and flawed sub that will actually make your system worse overall, owing to adding a cheap electronic circuit into the signal path, and bass that draws attention to itself.

The art of system building is compromise. This is even more important at the budget level. The two things that cost the most to do well are high volume and low bass. A budget system willing to compromise on high volume and omit the bottom octave can be incredibly captivating. Or you can have one that plays loud and goes deep but is utterly ordinary in every other way.

That’s the reality of the choices when you are on a budget. Which to give you some idea, I didn’t have a DBA until after Moab. So I would be in no hurry. My two bits.

If at all possible, you want to use an OB/Dipole sub with your Maggies. But that can't be done for $300. Check out the sub offered by GR Research (in collaboration with Rythmik Audio), and start saving!

Do an SVS SB1000 Pro now, and add another one when funds allow. Two subs much better than one, and anything cheaper would be an insult to your Maggies. You can get an SB1000 Pro from their Outlet for $575 including shipping and a risk-free 45-day trial…

https://www.svsound.com/products/sb-1000-pro-subwoofer

Through the crossover.

How will the sub keep the bass out of the Maggies?