I'm just as impressed rehearsing today as I was yesterday. Playing felt a lot more natural than it did yesterday, which probably means that as soon as I try anything smaller, it will sound lacking…

Hmmm, ... ... Have you tried connecting the Studio's Left (Mono) FX Send to the Input of your TCE BH550 (with the R115 cab)?? That way you might can get the RS40's possibilities along
with the fundamental and new tone you like! Of course, it'll take a bit of re-EQing on the TCE, like lowering the gain and whatnot, since the input is already getting a fully EQed signal. I do that with my RS40 and R500c for bigger gig volume. Could be the best of both worlds for you!

I thought about it. I know it can be done, but it looks like a huge chore, balancing two amps, having to get two mains, three devices... I'd probably rather get something like a Rumble 200 driven from the Studio 40 line out. But then the total gets dangerously close to Stage 800 and it doesn't make sense to faff about with two combos if one would do the job.
If the chaining I'm suggesting works for you tonally, then maybe you could just simply use the RS40 alone for practice and rehearsal!

I'll probably try that first. But as you said — if it works for me tonally. I'll have to try it. And the size would actually work for the rehearsal space, there's just enough space to stow the Studio 40 in there.
But then I'm facing two problems: I can't pin down the tone I want for the gig from the Studio, I have to go by feel. Having the head power every cab I use (assuming they're from the same lineup) makes more sense. And, Studio 40 was pretty expensive. If I resold it, I'd get pretty good money for it. Rumble 115 cab and TC BH550 head cost me total as much as the Studio...
On the other hand, if I prefer the Studio tonally, it then suddenly makes sense to get the Stage 800. Which costs waaay more.

... Well, [ahem] ... The Rumble 210
can be turned on it's side and ... Presto! It
is a side by side 210!! And
the R210 is no heavier than the R115, they are each exactly 39 lb.s. Then you can easily add casters to the new "bottom" side and a carrying strap to the other new "top" side.
Viola! [I mean, ... Voila!]
Maybe even
voilà Ǚ
Plus, don't underestimate the bass tones of the R210! I hear no significant difference between it and my Bassman 115 Neo cab!! At least try it with your TCE BH and see how it compares to your R115. It's true what they say, "tspeaker] size doesn't matter like it used to."

Then for big outdoor gigs stack your R210 on top of your R115 to get the full power out if your TCE! YOWZA!
Interesting. The black 210 is listed as being 20 kg (44 lbs) at Thomann. The silver one is the same 17 kg as the Rumble 115. I don't think the black one is heavier, must be a typo.
By "side by side", I meant arranged in a line. Fender's 210 is more boxy. Squatter but fatter than many 210 cabs.
Either way, none of the current cabs will fit in the space I have in the rehearsal room, not even the discontinued 112 V3. The earlier 112 V2 would fit, but I can't find any for sale.