It is unclear whether you have a problem or not, because the biggest risk is the TVC which has no specs.
That said, a suitable transformer would probably have a 15K primary. Here are three candidates:
1) Edcor makes a variety that are quite inexpensive and pretty good; I've measured two different examples.
2) The classic is the Altec 15095, available in a mumetal can with octal plug for connection
3) Magnequest makes a B7-15K
If it were me, I'd get a pair of cheap Edcors and see what you hear - be sure to give them 50 hours or more of breakin before making judgements! That way, you're not out much if is not the problem. If it does what you want then it's worthwhile to spend the big bucks on one of the others.
Be aware that a 15K:600 ohm transformer will reduce the signal voltage by 15dB. If you are using the FP-III switched attenuators, that's five clicks quieter.