help me select a single tube to "calm down" a preamp?

I have an Audio Research LS2B. It can get a tad "strident" in the upper mid on some recordings. I am now using a Siemens 7308 / E188cc and this is good, and often great tube for soundstaging and openess, but this tube also is a little noisy, mildly bright at times and is not the last word in bass performance. Would a Mullard suit the bill?.... How about an Amperex?....Your thoughts oh kind tube-0-philes :>)
audioman