DT50/HD500 User since more than 1 year.
first of all: my anwers to your good questions are subjective and therefore it is likely that you will get different ones to mine:
- If you want to have your tubes of the power section will take a bigger part on the overall sound you must not have your master volume knob pulled out (known as low volume mode), because therefore you replace the work of your hardware driven power section (with the corresponding power section tubes) by DSP.
- There is no general rule on what level of master volume knob and amp volume knob combination will be best for a tube driven sound which relates to all amp models inside your DT50, because the amp models react differently. For example if you use the Voicing II (PARK-75) you have to set the master volume level at least at 12 o clock to bring the power section into a saturation level for a creamy distortion sound. On the voicing IV you already get a tube driven distortion sound on a lower master volume level.
- For clean sounds it is always good to turn the master volume wide open to get a lot of headroom which helps you to avoid distortion on clean tones.
- What I do is: I set the master volume level on 2-3 o clock and my amp volume level to 12 o clock. This is a fairly loud volume level which I reduce with a power attenuator. This gives me in combination with the HD500 pre amp models a very good tube amp driven sound.
- If you replace your power section tubes in the DT50 they need to be rebiased. This is typically done by a specialized repair center. So if you are not familiar how to bias tubes in your DT50, tubes spares won't really help you in a giging situation.