Try the Psvane ACME 300B, that's a great pair of 300B tubes. To my ears, ACME 300Bs have a very open and precious sound, the combo of HA300 + ACME sounds like a mixture of high-end SS and 300B. I have tried TA300B, Elrog 300B, FullMusic 300B/Q, and ACME 300B recently on HA300, I'm confident enough to say ACME is so far the best tie with TA300B but in different flavors. I tested with TC, Susvara, D8000pro and Utopia+Dana all these headphones are handled extremely well under this combo.
Susvara: The overall sound becomes fuller and richer, the treble extension is further improved, the soundstage is about 30% larger than before. IMO, Susvara still fits this system the best.
TC: Previously, I do find the 300B amps generally make TC sound a bit too gentle, even with those TOTL 300B tubes (except Elrog). ACME is like a more refined or smoother version of Elrog (which sounds very clean but a little harsh in treble sometimes, an atypical 300B sound I would say). ACME is rich and warm in the bottom-end but clean and open in the treble. Thus, I feel ACME fixes some problems of TC without affecting the sound signature of TC by too much, the extra warmth in the mid-bass makes TC sound more "approachable". However, the bass attack is still not as satisfying as it with SS amps like headtrip.
Just wanted to say thanks for your post, as it became the tipping point leading me to decide on the ACMEs as my next pair of 300Bs. So now I have them plugged in, and they're sounding great straight out of the box, listening to them with the TCs. "Vivid" is the main word that comes to mind. I'm coming from EML 300Bs. I believe the ACMEs do sound like they're in a different class.