Best tube combination for SF line3 SE

Recently added SF line3SE to my system to match with Pass Labs X350 into the Revel Studio. However, the tubes came with it are the Valvo 6dj8, I am not too happy with the performance, hope to hear some Line3 owners' experience in tube setting as to improve its performance. Thanks for your inputs.
I also just bought an SF Line 2 last week and it came with Mullard NOS tubes which are supposed to be really good, based on what I've read. I wasn't happy with it either, until I replaced it with some cheap Sovtek tubes and the result is fantastic. Somebody told me that Sovtek NOS tubes for around $25 from Upscale Audio are much beter.
Did, either of you allow enough "burn-in" on your new linestages to eliminate that as the source of your dissatisfaction? I thoroughly enjoyed a borrowed Line 3SE for 3 months while hunting "the ultimate linestage" and although I ultimately purchased something else (Aesthetix Callisto), I still consider the Line 3SE to be one of the best Line-preamps I have heard. The Callisto edged out the Line 3SE in some areas that are particularly important to me but the SF had some pretty amazing virtues too. You sure wouldn't have guessed it was tubed by its noise floor. The Line 3 is quiet as a tomb, dynamic, has connection options out the wazoo, and the remote is great.

If you have a couple hundred hours on it so burn-in has been dealt with- perhaps a call to Andy at Vintage tube services would be in order.

Happy Trails!
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.