S.M.S.L SU-6 DAC ES9038Q2M XMOS DSD PCM768kHz/32Bit Bluetooth 5.0 LDAC USB Decoder (Black)

So, back to your original question for a moment: which sub will work with your Mackies? This one was just listed over the weekend in Hudson, OH: Velodyne CHT-8 listed at $100 (market value is $100; I would offer $80 and see what happens) This highly-rated sub, while compact, has plenty of umph to keep up with small nearfield monitors like the Mackies. More importantly, when using its preamp-level inputs and outputs, it applies an 85Hz high-pass filter to the output, which is ideal for your Mackies and most other speakers (see p5 of their manual). Combine your Mackies, this sub, and USB/Bluetooth DAC, and the upgrade to your system will be huge! A couple I would recommend are: Topping E50II ($200) SMSL SU-6 ($170) Both of these feature lossless Bluetooth, and I would not hesitate to buy either one used. Let me know your thoughts.