As you may know, NHT as a company has been going through many changes. They shut down and then re-opened the company. So maybe they cut out the sub because it was not a money maker for them. I have not kept up with their recent going ons as I worry about the longevity of the company as few have survived such transformations.My memory is that NHT used to recommend the S-00 subwoofer but it isn't listed on their website for sale now. If you were using some other NHT sub-woofer, I might learn how that that subwoofer worked with the M-00 monitors.