Sorry to dig up this very old thread, but I too am looking for information and schematics on the LSA Integrated Amplifier (Signature). I inherited one recently and there is a loud hiss when it's on with no signal. The amp works, but the hiss is about 50% of the sound regardless of where the volume is at.
I believe it's time to replace the tubes.
I have contacted Underwood Hifi and the local previous local importer here and neither could provide any assistance.
I'm hoping I can find some instructions on how to do this.