Thank you to everyone who has contributed wisdom to answer my questions - to pick up on a few points:
Dunno what the used market is like over there, but I'd say a 500 is prolly yer best bet. I have a 200 and with gain and master both ~10, it's LOUD.
As I've said many times, if I'd've known the daisy chain thing, I'd've bought 2 R100s.
Taking this as an example of the "keep thinking about a 100" school of thought, and noting the example of daisy-chaining cabs, my short and sweet response is:
interesting...
So basically I get one Rumble 100 to practice on, then use the FX loop to connect another one for another 100W if / when I need it, with the downside being lugging two combos rather than one (and splitting my cash on two lower-power amps rather than one biggun).
If your guitarist uses the Champion, and ever hopes to open it up perhaps outdoors, then you'll definitely need a Rumble 500 (350W alone) or a Rumble 800 combo (400W alone) to keep up. The Rumble 200 is just 140W alone, not much more than the 100.
...
Better to have the power and not need it, than to need it and not have it. And don't let child safety hold you back, just keep them away from the amps!
And the flip-side -
get a big amp - and it looks like the 500 is the main contender for this option since I don't need the fancy modelling stuff on the 200, so more power for the money rather than more features and 40W extra.
I was a 15" snob for decades since the '70s and had my doubts about the 2x10" Rumble 500 combo. But it has handled even the low B on my 5-string bass, even at high volume, just fine for 9-½ years, so far.
...
Sorry, if this isn't much help.
On the contrary - it was very useful, and your comment re: bottom B strings is exceptionally useful in this context, since I almost always play a 5 and I've been curious about exactly this point.
Thank you also to others who have responded.
What I think I have to do at this point is have a pint with the rest of the band and talk about what it is we're all expecting from the current setup (which is still pretty new - even if we play like we've known each other for years, which is very cool and I'm enjoying
so much

). Basically I've got to find out if we're doing the chasing gigs, audiences and taking it seriously thing - in which case the 500 looks like the right choice, long-term, or whether it's going to stay "for us" and be odd occasions where we're doing a bit of elevator music, improv. etc. as a now-and-then thing.
I'll try to report back when I've seen the chaps on Wednesday.
Thanks again all.
