Not really. All you will do is lower the volume on the speakers with the resistor in series. And it would need to be a reasonably hefty resistor; perhaps with heatsinking.
And Yes pushing that A+B button merely parallels the speakers so that the loading is of very low impedance.
25 years is pushing it as far as amplifier age goes and it may be reaching the end of its first life. Denon are usually very well built tho and perhaps an overhaul ?
However amplifiers are cheap for what you get these days and if you want streaming perhaps it is time to look at something new?
If those well known Aussie speakers are Whatmough they may well be even lower than 4R at some frequencies.
75 + 75 is usually no slouch and what I consider the minimum for bigger speakers.
Do you have a budget in mind? I like spending other peoples money for them.