re501 and se500 here
I can't see any difference in built quality. If you get solid unit, it's going to hold.
My 501 is in very good condition. I can't hear the tape splice (though that's function of the tape loop) and wow/flutter is very low. It's also low noise (in my opinion).
Soundvise I hear a lot of color from the electronics, which makes everything a bit midrangy (and moody). I'd go as far as saying the electronics is very colored, covering everything and not letting the raw tape sound (and distortion) to be heard. It's cool though, there is vibe to it. Spring reverb in my unit is a bit trashy sounding so I guess it's not the one that they used in re201s. It can sound great on the right material though.
SE500 has much cleaner electronics. Mine has some problems with the tape splice which can be clearly heard as a drop (that can stop the feedback) and depending on how tight the tension is set there is also a pitch change. I'm keeping it this way until I get tired of it, so far it has been fun because of some unpredictability that it brings. I like the tone much better than RE501, it sounds more like raw tape to me and it can get much wilder/agressive than 501 too. Motor speed change is very slow though, great for smooth transitions (when you bend the tone) but not so great if you want some rapid modulation on drums for ex.
Keep in mind every single tape echo will sound different, no two re501s will be the same. Too many variables.
What feels similar about all of them is some kind of fluidity in the sound. Something I can't hear in even very nice emulation like uad re201. The distortion also sound less muffled and more direct.
It's hard for me not to overuse them

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