Can I disconnect the speaker when I want to use the line-out (Rec/PA) only ?
No, with tube amps the amplifier will be damaged as a result. When a tube amp operates without a speaker, the internal peak voltages can become extremely high and permanently damage the power amp tubes and/or the output transformer. A tube amp should always be connected to a speaker or a power soak/dummy load . Tip: Pay special attention to good quality speaker cable and never use an instrument cable. A crackling speaker cable can cause your amp to break down.