Out of curiosity, in your TU8600, what are you driving the 300Bs with (12AU7/ECC82/12BH7) and what is your input gain stage (12AX7/ECC83)?
Have you done any other tweaking like output transformers (Lundahl), stage coupling capacitors (Mundorf, Miflex, Jupiter), power supply rectifiers (SiC in place of the stock Schottky rectifiers), bypassing some of the power supply capacitors and bypassing the volume control (instead using an external stepped attenuator)?
I too have an Elekit TU8600E (early version with bipolar ripple filter transistors).
Remembering that our definition of sound quality is quite subjective and varies between listeners, I will say that I'm quite happy with the pair of WE300B that have 1998 date codes (made at the long ago closed WE Kansas City works). Very smooth vocals, very nice bottom and not overly bright upper mids/highs. I am currently breaking in a pair of new production WE300Bs and like them very much. Similar to the KC works but a tad more defined sound on the upper end. Nothing was lost on the bottom end. I've also had Takatsuki 300Bs in a couple times for several months. They are also a good choice. They tend, at least in my system, to add a tiny bit more definition on the low-mids, a bit more "sparkle" in the vocals and well defined bottom end (BIG bass). Maybe a good word would be a bit more sterile sound but not at all objectionable.
Keep in mind that the items I mentioned in the first 2 paragraphs will greatly affect the sound no matter which 300B you choose. Some different brands of 12AU7s can make a huge difference in the sound quality. Or even swapping the 12AU7s with 12BH7s. The same goes for the 12AX7. You can spend a very eventful weekend rolling tubes looking for the sound you like.
And rolling the stage coupling capacitors (C103,203 and C104,204) will yield nice changes in sound stage and definition.