I think it’s going to be hard to find a tube warmer than the Mullards. Mullard has always had the reputation of being THE warm tube.
There maybe tubes that do sound warmer in your system but it would be a case of that particular synergy with your equipment. For example, the Amperexes mentioned above are not known as a warm tube and from my experience with them I wouldn’t call them warm, but they could, by chance, give you the sound you want.
I suggest you call Brent Jessee, Andy Bouwman (Vintage Tube Services) or Upscale Audio to see if they can recommend something for you. These guys are all very knowledgeable about tubes and will talk to you about tubes that may be what you’re looking for. Good Luck!
P.S. RCA Blackplates also have a reputation for being a warmer or smoother tube.