I had a Goliath III about 20 years ago.
I'm voting for you to get the 410. It sounds like you've got a system going that works for you right now - 210 for small stuff, 410 for big stuff. If it's working, then it sounds like going from the Goliath to the SL410 will just be a big upgrade to what you've got and like.
Also, if you get the 410, I think you could make a 610, and if you do have the TH700, it can handle the 2.67 ohms.
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I really hear what you say about one trip. I had a 4 ohm 212 for a long time. I really like having a single cab load in.
The only time it wasn't great was for the sometimes smaller gigs, so I got a 210. I hardly ever used the 210, but I was perfect for when I needed it.
A few years back, I got a matching 210, started gigging either one or both, and sold my 212. It's been good... but I'm going back to a 212.
I'll keep both 210s and have a few options. I'm guessing I will use the 212 90% of the time. I'll use the 2-210s if I know it's a big outdoor thing, and one 210 if I know it's a small stage.
Most guys on TB seem to think the "modular" rig is superior... after doing it for a while, I'm going back to one cab.
IMO, this is the
benefit to option 1 (2-210 cabs) -- you can bring both cabs, and make a call about what to use at gig time.
For example: a couple months back I had a gig and was expecting a large outdoor stage. When we got there, they said it was too cold, and moved us inside, into a pretty small barroom. So, I only used one of my two cabs.
Another example: when I was younger and did a bit of touring we would sometimes be on a huge outdoor stage on a Friday night, then in a tiny club on a Saturday night. Having two of the same cabs in the trailer would be perfect.