Bill Fitzmaurice

My advice is don't double post. 🙄

Absolutely correct. The second cab will increase both sensitivity and maximum output by 6dB. That's the equivalent of quadrupling power. Chances are the 350 watt amp will deliver all the power the drivers can handle. It's the rare twelve that can make use of more than 175w before running out of excursion capacity. OTOH the number of twelves that can take 500w without exceeding their excursion capacity is small, and they're PA subwoofer drivers, not electric bass drivers.

I've been trying to call my local paint supply store but can't get a dial tone on my phone. 🙄

Awaiting the first person to ask 'What's a blackboard?'

This is a proper tweeter for electric bass: https://eminence.com/collections/bass-guitar/products/bgh25_8 Don't be thrown by the 25 watt rating. That's for the actual power in its passband, which is on average only 8% of the full system power. All tweeters used to be so rated, but it became too difficult for people to comprehend that of a 300 watt program perhaps 25 watts goes to the tweeter. In the cabs where I specify this tweeter I use a 4th order high pass filter that doubles its power handling.

You can't do that. BTW, the usual reason for blown tweeters is that the crossover is dodgy, and does not provide adequate protection from low frequencies. They also tend to use cheap tweeters. You may be able to upgrade it, but no one can say how without knowing what's in there for both the tweeter and crossover.

Not really. The product of cone area (Sd) and excursion (Xmax) is displacement (Vd), which combines with sensitivity to give SPL. To further complicate matters driver size, and in the case of multiple drivers layout, affects dispersion. There are no short and sweet rules of thumb, other than always use the same drivers in a grouping, and always have that grouping vertically aligned.

If your amp is 2 ohm capable a BF Super Compact. That gives the option of running 1x12, 2x12 or 3x12 depending on the gig.

This is a case where 1+1 equals 2, so long as the drivers are identical and the box net volumes and ports are identical.

If it's bi-amped the crossover will prevent lows going to the 210. When not bi-amped turn down the bass EQ.

That wouldn't be affected by the cab height, as low frequencies radiate omni-directionally. Mids and highs are directional, so the cab must be higher to hear them well.

Even at 125Hz 'A' weighting is 20dB off from actual. To bring that into a more meaningful context 20dB is the difference between 10 watts and 1,000 watts. 😳

If it was high frequency feedback that's a possibility, albeit a slim one. If it was low frequency feedback it was just too loud.

Not really. You can get by with a C weighted meter, for instance: https://www.amazon.com/Professional-Backlight-Accuracy-Measuring-30dB-130dB/dp/B01MZ0IUGY/ref=asc_df_B01MZ0IUGY/?tag=hyprod-20&linkCode=df0&hvadid=312357852128&hvpos=&hvnetw=g&hvrand=7748314164138405611&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9002322&hvtargid=pla-348660547600&psc=1&tag=&ref=&adgrpid=62539486699&hvpone=&hvptwo=&hvadid=312357852128&hvpos=&hvnetw=g&hvrand=7748314164138405611&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9002322&hvtargid=pla-348660547600 I was hired by a major outdoor concert venue that was being threatened with imminent shutdown because of noise complaints from the neighbors, despite having already paid a 'professional' sound control company $50k for a sound metering system. The complaints were about excessive bass up to two miles away. It took me all of five minutes to see what their problem was. The system was 'A' weighted. According to it the levels off-site never exceeded 75dB. But it didn't measure bass, so the actual off-site levels were routinely hitting 105dB and more below 100Hz. I wrote a new sound control protocol for them, then attended their concerts for three years to ensure compliance. It paid me better than playing, so I had no complaint. The noise complaints went away as well.

dB (A) is useless for music, it doesn't measure bass. You must have a dB (C) meter at the minimum, dB (Z) being preferred. https://www.noisemeters.com/help/faq/frequency-weighting/

In the case of under-hung voice coils the calculated xmax was zero. That caused more than a bit of consternation for loudspeaker designers. Eminence was one of the first to adopt Klippel. Circa 2004 they had to switch voice coil former suppliers, as DuPont stopped supplying Kapton to the industry. Changing the formers changed the driver specs, so they had to measure them all with the new formers to update their data sheets. They had recently acquired a Klippel analysis rig, so from that point used it to measure xmax dynamically. Most manufacturers have since followed suit.

All drivers exhibit this behavior if sufficient power is applied to push the cone past xmax. However, at higher frequencies the voice coil may burn out first. Even when present one might not be aware of it when it's not severe. The fact that speakers have a point beyond which more power applied only creates higher THD has been employed by Klippel GmbH in redefining how xmax is measured. It used to be calculated by comparing the voice coil length and magnet top plate thickness. It's now the excursion at which THD reaches 10%.

That's not only not unusual, it's common. How sound waves work isn't the least bit intuitive, so it's a field where you must be taught. Figuring it out on your own isn't impossible, somebody was the first to do so, but it's not easy.

Yes, assuming the amp can handle the halved impedance load.

Boundary reflection sourced cancellation. It can boost it as well, via cabin gain. I'm afraid not. What is happening is that the cancellation modes that exist within the walls don't exist outside of them. It's the exact same reason why volume on or near the stage can be less than at the back of a room. That phenomenon gave rise to the myth of wave propagation. Running out of driver excursion will do that, but not the room. Continuing to crank the volume results in increased compression and THD. It won't hurt an amp, and may not hurt the speaker. It's how guitar amps get a sweet cranked sound. Guitar drivers are made with a short excursion, xmax, to make it happen with as little power as possible. Bass drivers have long excursion to prevent it from happening to the greatest extent possible.

It will, by up to 6dB with another identical cab depending on the particulars. That's the equivalent of quadrupling power.

They still are. 😉

All things considered if I had to settle for store bought I'd have a pair of 8 ohm 210, stacked with the drivers vertical. When two aren't required leave one at home.

Whatever. I still use real measurements that we got from good King Henry VII, not that latecomer Napoleon.

Neo cabs aren't necessarily lighter. For instance, one of the most common tens is the Eminence Beta 10. It weighs 3.1 kg. The neo Deltalite II 2510 weighs 2.1 kg. In a 410 the driver weight difference is only 4 kg. Neos are quite a bit lighter when you're talking about premium drivers, but most bass cabs don't use premium drivers. It's a different story with the cab material. A 120 mm x 240mm sheet of 18mm Italian Poplar weighs 21 kg, the same size of MDF in 15mm thickness weighs twice that. You have to look at the weight of the finished cab, not just the driver magnet material.