There's almost always a way to adjust bias voltage, even if it means changing out a resistor. Or put a 20K trimpot in series with that 27K resistor. In any case I've used JJ KT77 and had no problems, ordinary wear & tear over the course of years. And New Sensor offers their Gold Lion version if you want to pay a lot more. Push comes to shove, I have an original pair of unused British Genelex Gold Lions, I'd love to see $700 for. Ka-ching!