#1
I run a re-issue Fender Deluxe Reverb (has an internal 8ohm Jenson) with a second cab (Mesa 2x12 with Vintage 30s - also rated at 8ohms). I understand this brings the total draw to 4 ohms, but I've heard that this this type of amp is somehow able to 'adjust' itself to accommodate this. Does that make sense?

Should I be worried about wearing out my amplifier's Tubes or Output Transformer?
#2
Just run the cab by itself. If you run a 4 ohm load on an 8ohm amp you'll probably damage it. I know mesas are meant to tolerate it (hell they can run missing tubes) but I don't doubt that amp could.
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#3
some combo amps i've seen bypass the internal speaker if an external cabinet is plugged in.

you may not need to worry about that if the deluxe reverb works in this way.
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#4
In general, I've heard of Fenders being ok with running a mismatched load, however, regardless of whether it will damage your amp or not, it will almost certainly run your tubes to the ground.