16 or 8 ohm Replacement Speaker

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I had always heard that with most tube amps you could safely mismatch speaker impedances by "twosies" — like connecting a 4 or 16 ohm speaker to an 8 ohm tap. Yes, it would not be the correct load on the power tubes but it should be "good enough for rock'n'roll" and should not damage the speaker or OT. Reality check: Do amp techs today still believe that or have they learned otherwise?...

Yes, I think that guideline is probably generally ok if used with caution, with the proviso that it's not a copper bottomed guarantee / applicable to every tube amp.

When working as intended, many guitar amps push their tubes beyond at least one of their limiting values; if cranked into a load that's too far off nominal, the limits can only get stretched so far before the stress damages the tube, with possibly significant potential for catastrophic failure / collateral damage.

Beam tetrodes with reasonable operating conditions tend to be fairly resilient (what's the Marshall's HT?), so I think the DSL15 (with 6V6) should be ok into a load impedance down to half nominal.

I would be more reticent if it was pushing pentodes hard, eg claiming 20W from a pair of EL84.