Which power supply are you using with your preamp? If you are using the PS1T or the PS2, you have a 6x5 rectifier tube for the high voltage. There is a separate transformer for the 12volt heater supply. The tubes will glow if they are getting the 12volts, but there will be no sound if the rectifier tube is bad. The fix could be a new 6X5 rectifier tube.
I have the Line 2M with the PS1T power supply. I actually have the Line 2M that is reviewed on the Mapletree site. With the Dr Lloyd's help, I moved everything to the larger chassis and configured it with 6 inputs and 2 outputs. I upgraded to Auricaps and Black Beautys. Volume control is an Alps and the balance control is an Alps Black Beauty. Selector switch is an Electroswitch.
I recently, again with the Dr's help, started using a Bendix 5852 rectifier tube in the PS1T. This was a large improvement over the 6X5. If you have a PS1T, you only have to change one resistor to use the 5852. I would highly recommend this change.
If you have a PS2 power supply, you cannot use the Bendix 5852. Repeat, you cannot use the 5852 in a PS2 power supply. It is a different design from the PS1T.
The bottom line is that the Line 2M is an amazing preamp. You only need three tubes-two 12sn7 and one 6X5-to have a fantastically musical sound. All the detail is there and the sound is so pleasing. I had a $3000 Coda preamp when I bought the Line 2M just to play around with. I sold the Coda shortly after and I've never regreted it.