I prefer using a Booster pedal rather than an Overdrive-with-clipping pedal. I like this chain, for hearing *only* power tube and speaker saturation:
EQ pedal with bass cut, mid boost, treble boost
Booster pedal that outputs a high voltage span without clipping
Power tubes (bypassing any preamp that might introduce preamp clipping); can usually go in through the effects return jack.
Low-wattage guitar speaker, driven directly with no attenuator, and mic
The goal is to know and control how much preamp clipping and how much power stage saturation you are getting. Most amps have no way to be sure of your blend, and you cannot get intense power-stage saturation with no preamp clipping. The old, non-Master-volume Marshalls can do that, but supposedly with the Master Volume Marshalls, the preamp insists on clipping, before the power tubes are driven very hard -- at least that's the implication, in the way people talk about it. They say that the non-Master Volume amps can get "more" power stage saturation than the Master Volume amps.
It's usually trivial to install a jack to inject a signal directly at the main volume pot of a tube amp, thus bypassing the preamp and avoiding any chance of its clipping getting mixed into the equation. Some sort of transformer or capacitor might be needed, but I've done this without such a device.
Once I can get extreme 100% power stage saturation, then I can reduce it and mix it with a known amount of preamp clipping. This gives better control than being forced to have some mixture of preamp clipping with your power stage clipping.
If your amp has two channels, you can plug into the clean channel as usual and hit it with a massive, nonclipped signal, and hopefully avoid preamp clipping, while thoroughly saturating the power stage.
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