• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

tube/solid state hybrid

The Magnatec web site is http://www.magnatec-uk.co.uk
and they seem to sell online.

As far as replacements, the BUZ900P/BUZ905P sould be prity mutch straight interchangable with most of the power mosfets commonly used in audio, you shouldn't need to change the circuit at all.

Some devices you could use are,
Hitachi 2SK1058/2SJ162
Exicon EC10N16/20
International Rectifier I am not very familiar with, but something like IRFP240/IRFP9240 looks about right.