Here's my new Aria Pro II SB-900 (May 1980). The previous owner warned me that the tone knob does very little (it's V/V/T even if the 1980 catalogue specs state otherwise, the knob and switch count match the catalogue photo at least).
Further testing reveals that the output isn't great (but not terrible) on either pickup and there's a fair bit of hum and hiss. My 1983 Aria Pro II CSB-380 with a single MB-IV seems to have double the output and is not as noisy.
So, it looks like it's pickup replacement time, though I will get this double checked by my local bass-maker. The 1979 & 1980 SB-900's appear to have had MB-I pickups and from 1981 onwards MB-II's.
So, question is, do I ask Veijo at: https://www.rautiaguitars.net/aria-pro-ii.html for a couple of MB-IE's to stay close to the original MB-I's or do I put in a pair of MB-II's (as the SB-900's had, post-1981).
I'm guessing the more pole pieces (MB-IE) the better for a passive bass like the SB-900? http://www.projectguitar.com/forums/topic/47441-aria-pro-ii-rsb-600-restoration/
Found this commentary here. Rautia's MB-IE's come with a 4 wire option though:
https://www.basschat.co.uk/topic/255626-aria-sb1000-noisekiller-circuit/?page=2