Of those models you listed, 5000, A and X are all capacitor coupled. These have their own distinct sonic signature, and noticeably different from direct coupled units. The cap coupled sound of the late 60's Sansui gear is typically described as warm, with tight punchy bass and forward/strong presentation in the mids and highs.
I - and many others here - are quite enamored of it, but I understand there are those who are seeking accuracy and lack of coloring in the sound above all else. If you fall into the latter group, these models will probably not suit your fancy. The 5000 in its three variants does not have a relay protection circuit (they are fitted with an SCR triggered protector and not a particularly great one at that) and the original driver boards were failure prone on many units, but excepting those two points, overall build quality is excellent.
Dial scale is only lighted during tuner operation, but is not affected with tape monitor loops being engaged. Phono and Aux modes do black it out however. The 5000 and A have a tape head input, intended for use with R2R decks lacking an integral head amp. These are a relic now, and so this input is unusable unless one removes the head amp from the signal path, whereby it is usable as another Aux input. The 5000X omitted the tape head input and replaced it with a second phono input. The dial scale appearance on the X also received a slight cosmetic refesh.
881 has, if I have understood correctly, the same basic amplifier topology as the 9090 series that came after it, just with less output power. The 881 series is also very well liked by a large contingent of Sansui fans. Again though, given their age and relative lack of technical advancement compared to what Sansui was doing circa 1979-80, more critical listeners may not find them up to their needs. I don't - and haven't yet - owned an 881 series units so my knowledge of them is limited. I'll leave others to fill in the finer details there.
I will say that Sansui entered what is considered the modern era of amplifier design (DC, very high slew rates, very low TIM) around the introduction of the first gen G series receivers and the AU-919 integrated amp, which is the 1978-79 time frame. Everything prior to that was evolutionary in design, and then there was a sudden big jump in amp design sensibilities that constituted a more significant change in their sound going forward.
For those who prefer the ultra clean, uninfluenced sound produced by modern gear, 1978/79 is probably as far back as you can go and still be satisfied. Prior to that, there are characteristics of the Sansui sound that will probably not impress hardcore purists of the sort who are seeking "a wire with gain" type performance, and have a distaste for tone controls and the like.
Maybe there are some models I never heard of? Would like to know which are capacitor coupled and which DC coupled, also relay or protection circuit but mostly the sound. Oh, also do they all light up the scale only when switched to radio tuner as eight does?
