Without getting into all kinds of further permutations of ways to 'improve' this system, my advice FWIW is to go with plan B and give back Mom what she wants. It's your safest, simplest, and ultimately cheapest course of action given the situation, and nothing else you could try to do even with the best of intentions will probably result in more satisfaction on the part of the owners. It seems as if the sound they were used to was dependent upon utilizing the loudness contour and tone controls of the Crown full-time, and that the new amp presumably has more gain than the old amp, which didn't highlight the hiss from the old preamp so much. Quit while you're still within earshot of being ahead, and beat yourself a hasty - if wiser - retreat to the status quo.
As for the speaker hook-up polarity deal, to get the change you observed you need to have reversed only one speaker's leads, which would put the two speakers in-phase with each other if they had been out-of-phase before, resulting in stronger bass response, as well as correct stereo imaging. Changing both speakers' hookup polarities at the same time will reverse the absolute phase of both speakers together (something not audible for the purposes of this discussion), but would not affect the relative phase congruity between the two speakers. This all tranlates as, just make sure the two speakers are hooked up the same way as one another at both the amp and speaker ends (+ to + and - to - *or* + to - and - to + for *both* channels), or if you can't easily look behind the speakers, just go with your ears.
As for the speaker hook-up polarity deal, to get the change you observed you need to have reversed only one speaker's leads, which would put the two speakers in-phase with each other if they had been out-of-phase before, resulting in stronger bass response, as well as correct stereo imaging. Changing both speakers' hookup polarities at the same time will reverse the absolute phase of both speakers together (something not audible for the purposes of this discussion), but would not affect the relative phase congruity between the two speakers. This all tranlates as, just make sure the two speakers are hooked up the same way as one another at both the amp and speaker ends (+ to + and - to - *or* + to - and - to + for *both* channels), or if you can't easily look behind the speakers, just go with your ears.