Yeah for sure, you’ll definitely get phasing if you go into the standard input because the second preamp will introduce some phase delay for sure. If you use effects send/return you’ll probably still get phase shift from the power amp too though perhaps less (unless the effects loop is pre-preamp, but that would be dumb.)
Personally I would absolutely go for a bigger amp and multiple cabs over this idea. But like I said, I’m also not a real amp expert.
Good point, @howard and @JoshFossgreen . . . I didn’t think of that.
Thanks for your input, guys . . . 
Cheers
Joe
The power amp stage (any amp really) will introduce a phase shift. The preamp will introduce a serious one.
Another thing I am missing here is why you would want to do this. It seems like categorically the worst option. Even if you don’t want to buy a big bass amp and cabs, instead of this you can get 2000 watt powered PA speakers for $250 each. Just run your tone-coloring preamp into a couple of those instead. Mackie makes a 1300W 15" one for $350:
But really, just getting a less expensive, modern version of something like the 800RB you used to have seems like the best option to me
Even used G-K modern amp heads aren’t too bad. Used MB500’s run $300-400. And G-K is on the spendy side of course.
I agree Dave, I’m making the assumption that the cabs - if separated - will not be 30+ ft apart…I’m more thinking about either side of the drum riser…thanks!
Hey Josh! With all due respect man, the primary amp’s effects ‘Send’ is connected to the secondary amp’s effects return, effectively turning the secondary amp into a power source only, no ‘coloration’, pre amp interference, or nightmare scenarios…same tone from the primary, amplified by the secondary…power amp boost only, and we’ll be talking about using identical combo amps too…you know…cheap, used ones…
And when we say ‘small’, I’m thinking if I go woth the Promethean, THAT’S small…but the Acoustic B200 MKII is NOT a small combo, and 2 of them…
I think we may be fretting ( get it?) over stuff that will be minor in scale (…again) compared to the benefit of output boosts, and THAT’s what I’m trying to do… 2 small combos working together to fill the stage/room with low end…
and Daisy chain sounds kind of fragile, while SLAVING sounds like someone’s in charge!
sorry terb, I’m missing the part where you mention ‘2’ returns…
we are going to try 2 small, cheap, used combos, running the FX Send from one, into the FX Return of the second…2 amps only
thanks again Howard…but the pre amp of the 2nd combo will not be a part of the signal…FX return will turn the 2nd combo into a power amp only, with the signal from the 1st combo…no ‘phasing’, a clean signal boosted by a second amp…
as for why…I can only afford to get a relatively inexpensive combo to start with, and down the road ( month or two) look to add it’s twin, so this works for me…now…that’s all there is to it.
and once I’ve got the first combo and it’s slave, I’ll report back! Are there other ways to fill a room with low end?..of course!.. it’s an idea born out of necessity…if I had hung on to my touring rig, I wouldn’t have had the reason to mention it at all!..but…this is where I am now bass dudes…
I really appreciate all of the input and suggestions…even the pushback! thanks!
OK, @backlineguy . . . so you’re going to use a standard 1/4" jack instrument cable to connect two combo amps together, from the “send” jack on the first amp to the “return” jack on the slave . . . this bypasses the “gain” on the second amp, right? 
As for why I’m interested in this? I wish I could say I’m going to be gigging with a band, but I’ve just been wondering if it can be done! 
. . . ALSO because I have a Rumble 100 and would rather expand it, rather than sell it and buy a head with separate cabinets . . .
Cheers
Joe
Hey Jazzbass19, I’m no expert, let’s get that out of the way…so if the ‘Gain" is the level control of the slave amp’s preamp section, then yes, I believe so…if the input to the slave is the FX return, and NOT the slave amp’s Input jack, then we circumvent the slave amp’s preamp section…more modern amps - with FX loops or ‘send’ and return’ for effects, allow this relatively easily, whereas if the amp is older and lacks the FX loop, it gets more complicated…I found this article from Seymour Duncan, and I think it’s what I’m after, although they are discussing guitar amps, the principle should hold for bass amps too…
Once you plug into an amp’s send jack, you interrupt the signal and silence the first amp. You would need to use the DI out of the first amp. On the Fender Rumble series the DI is affected by the master level knob, so both cabinets would change volume together.
The return jack usually inserts back In after distortion and EQ blocks. For a particular amp look at the block diagram in the manual. Everything on the block to the left of the return jack is bypassed. Everything to the right of the return jack will affect the second amp’s signal.
Stacking (2) 100 Watt combo rigs doesn’t get into earth shaking territory. It’s possible that if 100 Watts isn’t enough, then more than 200 may be necessary. Hard to say without knowing the scenario.
It may be worth buying the cheapest big amp head you can find and then upgrading cabinets as budget allows. The Harley Benton block amp is 800 Watts for $300 and drives down to 2 ohms. I would start with that and the cheapest used at all reasonable sounding 15” 4 ohm cabinet I could find.
Every modern manufacturer uses the same flat transparent power stage circuit topology in their amp. The “sound” comes from their magic tone controls and overdrive on the preamp. A cheap clone or used pedal can supply this and can be upgraded as budget allows.
huh? it’s the FX loop…silence the amp? It’s not like a headphone output…THAT would silence the amp…
The loop is completed when plugging back into the effects return of the loop. That’s what brings the signal back into the amp again on most amps.
An amp could have a blend control to the effects loop and then it would be possible to keep the dry signal internal to the amp an also have a cable plugged into FX send. I think that’s the rare case, but it can be confirmed on the block diagram in the manual.
It’s easy enough to test. Try it.
yeah it’s pretty much the difference between a serial and a parallel loop. serial loops are way more common.
Willing to bet the Fender loop is serial and, knowing them, after the master volume too 
Ah, gotcha, that makes more sense! Still not how I’d work a setup, but if it works it works!
I have this combo amp and I like it a lot.
I spoke to Fender…( what a concept)…slaving these combos IS an option using the FX loop, and both amps will produce output…at least in THIS case, the Send output will not cut off the speaker’s output…
But I’ve decided to try the best of both worlds. I now have a used, sub $200 Ampeg B2R driving a used, sub $130 GK 410, and instead of simply adding a 115 cab to this, I’m using the FX Send to a used, sub $150 Acoustic B200 MKII 115 Combo…low end for days, plenty of headroom to keep up with any drummer…TWO drummers!..and a relatively articulate sound eminating from my 410…who knows, at some point, I may even add another used 115 cab to drive the Ampeg down to 4ohms and use it’s 350watts…
anyway, thanks to all who posted here, shared their support, know-how, comments, dismissals, questions, etc…now looking forward!
It’s an interesting choice and I’m glad it’s working out for you.
It sounds like you got the 8-ohm 410? So you now have 175W going in to the 410 and either 100 or 200 going in to the 115 (depending on if the Acoustic delivers full power to its internal or not)?
I’m wondering about the real difference here between this and just fully driving a 410 and a 115 with the B2R.
