I also own a Line 3SE with all Sieman ECC88, it was one of the last one made. I used to own the Pass X350, the combination never sounded alive, linear, nor magical to me. I suspect the problem is more amp matching than the tubes in your Line 3SE. I heard very lean midrange with no bass most of the time. I did tried other tubes and interconnect, general result was similar.
I also tried a PS Audio HCA-2 for fun, I thought the result was better than the Pass. Bass bloom a lot more and midrange did not sounded as "recess". Top end couldn't match Pass, inner detail couldn't match Pass either, but overall enjoyment level was higher.
I am still looking for that perfect amp to match my Line 3SE. So before you toss your Mullard, try a different amp.
I also tried a PS Audio HCA-2 for fun, I thought the result was better than the Pass. Bass bloom a lot more and midrange did not sounded as "recess". Top end couldn't match Pass, inner detail couldn't match Pass either, but overall enjoyment level was higher.
I am still looking for that perfect amp to match my Line 3SE. So before you toss your Mullard, try a different amp.