Well, I have some good news and some good news on the sparking (and capacitor) front(s). I finally solved the sparking issue. When I first retrieved the 1600, the aluminium cover panel (the brushed plate surrounding the tape turntables) had the two brass locating pins pulled out of their retaining holes...

Thanks casdave, that's exactly what's happened. The tants (especially on the preamp boards) have long since turned into, er, dynamic resistors. I changed the two in the preamps, and the hum and crosstalk disappeared from the preamps. I've just got around to getting a big bag of good quality...