I only have a 4B-SST² for my Magnepan 20.7s... I use a Pangea AC9SE signature for it ($250). Other cords I would consider are the PS Audio ($1K) or the AudioQuest Hurricane. ($1.5K) The Hurricane has been getting plenty of raves.
Another thing to consider is the duplex outlet in the wall. (If you cannot get dedicated AC circuits, at least the duplex in the wall can be upgraded.) If you still have the one there when you moved in... replace it. Even a Pass & Seymour heavy duty ($6) will be an improvement.
All this is if you can hear it.. If you find no difference. Then you can save plenty of money.
I noticed only a small change in amp behavior when going between to wall and to my Furman REF 20i conditioner high current outlet. Adding high end duplex made more of a change, and made it easier to hear differences in powercords too.
Thank you Elizabeth!
I do have a dedicated 20 amp circuit for the listening room. I also installed Pass & Seymour "hospital grade" outlets. Bright orange so I remember which outlets are on the dedicated line. The receptacle wipers really grip the plugs tightly. Far better than the 78 cent contractor grade found in the rest of the house.
All of my gear is plugged into this circuit including my pioneer elite plasma monitor. It is a connected via a Siemens surge protector. This is disconnected for serious listening. It adds a bit of hash to the sound even if switched off.
Digital stuff is unplugged when I play records, but I rarely play digital sources any more. My Well Tempered Amadeus-Grado Reference Master-Hagerman Trumpet with vintage Mullard tubes kicked digital out of the listening room.
The display of my Bel Canto Pre 6 is turned off. All of these actions improve the sound by a small degree but it's noticeable.