Inexpensive sub for MG12's

I would try to listen to the Martin Logan Dynamo 700 or 1000. Audiophile Liquidators currenly has the 700 listed for just under $500. I'm pretty happy with my 700 and I think it crosses over all the way down to 35 Hz.

Bwp - I purchased my Dynamo 700 from Audiophile Liquidators and had an immediate issue with it (a problem with the wireless transmitter). They were very friendly on the phone and had me sent it back for testing. In the end, they shipped me a new sub. Every time I spoke with customer service it was the same person and he remembered me from previous conversations. I would purchase from them again based on my experience.

Depending on the type of cross over you're dealing with, the subwoofer will reproduce frequencies above the cross over setting and the speakers will produce frequencies below the 45 Hz just at lower volume levels. I think it's possible for an electronic crossover, think audyssey type stuff, to completely cut off frequencies, but I'm not sure about that. I think my Martin Logan references setting the crossover at 75% of the rated bottom end of the speakers for a starting point to adjust from.

Focal rates my speakers for 40 Hz to 28 kHz with a low frequency point of 33 Hz. I have a test tone CD that I tested and they were able to reproduce just under 30 Hz, but at a much lower volume. I don't know if your speakers are similar or not.

The ML dynamo 500 is ported and the 700 and 1000 are sealed designs. I don't know about their higher end subs.

I have the Dynamo 700 and rarely use it above 1/4 volume in home theater mode. When I had it connected to the main channel pre-outs the volume was even lower. I was amazed at it capability when it first arrived. It's not the greatest sub on earth, but considering it's size and price I'm impressed.