Ampeg advice!

I'd listen to both cabs and buy two of the one I like better.

 

As @el murdoque indicated, the best recommendation is to try both to see which you like best, and then get a pair.

The only way to find out of the 112 and 115 will play nice is to try them. If there is too much phase and group delay variance between them, they will tend to fight in a way that results in degraded sound.

112v Specs
RMS Power Handling: 300 Watts
Frequency Response (-3dB): 70 Hz-18 kHz
Usable Low Frequency (-10 dB): 35 Hz
Nominal Impedance: 8 Ohms
Sensitivity: 96 dB
Maximum SPL: 121 dB
Dimensions (W x H x D inches): 24 x 17 x 16
Weight: 45 Pounds

15E Specs
LF Drivers: 1 x 15” Cast
Voice Coil: 2.5”
Magnet Weight (each): 80 oz
RMS Power Handling: 200-Watts
Frequency Response (-3dB): 50Hz-3kHz
Usable Low Frequency (-10dB): 33Hz
Nominal Impedance: 8-Ohms
Sensitivity: 98dB
Maximum SPL: 123dB
Dimensions (W x H x D inches): 24 x 22 x 16
Weight: 72 Pounds


Here is how I would compare the specs:

Both cabs are 8 ohms. This will allow you to run both on an amp rated for 4 ohms minimum. The SVT 4-Pro can be bridged for 1,200W at 4 ohms. This would make 600W available to each cab, which is way over their power ratings. One side of the 4-Pro is rated at 490W at 4 ohms. This is still probably enough to blow a pair of either cabs if you really crank the lows and push the amp to it's limit.

The 112AV has a higher power rating, but the 15E has a higher sensitivity rating. The 15E also has a higher Max SPL. If you feed a pair of 112AVs 600W total, the max calculated SPL should be 127dB. If you feed a pair of 15Es 400W total, the max calculated SPL should be 129dB. Either is more than most people need, but some require more.

F3 is 70hz for the 112AV VS 50hz for the 15E. F10 for the 112AV is 35hz VS 33hz for the 15E. I could be wrong, but this suggests the lows start rolling of sooner with the 112AV, but the slope is not quite as steep. I am not an expert on cab tuning, but this may suggest significant variance in phase and group delay.

Both cabs share the same width and depth. The 15E is 5" taller and 27lbs heavier.

I checked prices online and the 15E street price appears to be $50 higher.

The 112AV is smaller, lighter, and less expensive. If I tried both cabs and could not choose the one I liked best, I would go for the 112AV unless I needed the absolute highest SPL possible. Keep in mind the max SPL difference is only 2dB which is generally considered insignificant. I would buy the 15E if I found it's sound significantly more to my liking.

Good luck!

 

Both of these cabs are 4 ohms per the spec sheet. The V4B has a 2 ohm tap so you can run both cabs. The 212AV has a 2dB higher sensitivity rating. The 212AV has similar low end to the 112AV. The 18 has a much lower F3 and F10. The difference is even more extreme than the last combination of cabs. If you can find a 212AV I would suggest taking your 18 to the shop and actually trying the combination out.

I used to run a 112 combo with an 18" extension cab. The tuning of each cab was significantly different. Adding the 18 allowed me to get loud enough for the gig, but I would not say the combination of cabs produced great tone. I used this combination because it's what I owned, and I did not really want to spend cash at the time.

 

In your OP you say you're looking for a smaller rig. The V4B is a big head. It's not as big or heavy as an SVT but it's still a big amp.

The various mixed-and-matched cabinet options you're suggesting might very well work but it's kind of a crap shoot. The best way to take advantage of multiple cabinets is to use matching cabs.
It's not that mixing cabs can't work, it's just that you never really know until you try and the odds are good it won't be ideal. Using multiples of the same cab doesn't only offer the advantage of predictability when you use them together; the other advantage is that you'll have the same sound when you only use one cab (for small rooms) as you do when you use the full rig.

If you use a pair of 115 cabs and a V4B your rig will not be significantly smaller than your previous rig. I would suggest going with a pair of the 112s and then find a smaller head that you like.

 

I am not saying the none these combinations will work. I am saying you will have to try them and decide for yourself. There is a risk you will spend money and not like the results, but it doesn't have to be a lost cause given your rather impressive dreams.

Both combinations of cabs will work with either the V4B or SVT 4-Pro. On the V4B the load of the SVT212AV and SVT18 will be 2 ohms. Two 4 ohm cabs in parallel = 2 ohms.

If you run the SVT 4-Pro in dual mono, each side can run at 2 ohms. So you could run both the SVT212AV and SVT18 on one side of the amp. This will split 600W to both cabs, so each cab gets 300W. If you run one 4 ohm cab on each of the output sections, the amp is rated 490W at 4 ohms. So in this configuration each 4 ohm cab gets 490W.

 

You can mix any head with any two cabs as much as you like.
The only thing you need to watch out for is the impedance. Never run SS amps below their minimum impedance and don't run tube amps above.

Apart from that, feel free to try whatever you desire. Couple a pair of cabs that can handle 1200W with a 5W head? That's what I do at home. Put a 600W amp on a cabinet that can handle only 250W ? I gigged that - just be reasonable with the master and the bass knobs.
Have a grossly mismatched set of a 4x10 and a 1x15 ? Well look around, stages are full of that.


The one safe thing to say is that when you couple a cabinet with a second, identical cab, you'll have the same sound, only more of it. The bottom end gets a lot more substantial and the overall volume is raised by 3dB through doubling your speaker surface and by another 3dB if you're using a SS amp and your wattage doubles through halving the impedance.

6dB does not sound much on paper, but it is a world on stage.

Pairing mismatched cabinets is always a shot in the dark and only personal testing can reveal if it is on target or way off.
Someone else's experience means they played in their room, through their bass/pedalboard/amp settings with their technique in their mix of their genre of music. Some - or all of it might not translate 1:1 to your environment.

I've coupled drivers throughout my youth when I did not have the funds available to buy the rig of my dreams - or even a decent one. Some combinations were surprisingly good while with others, I ended up disconnecting one cab to get the best possible sound. It's all theory unless you try it.
The only safe bet is identical cabs and cabs that were engineered to work together.
If I recall correctly, Mesa Subway cabinets are done that way and you can combine any two.

 

The information you are reading about regarding mixing and matching cabinets is TRUE.

For example, I mixed an Eden 410XLT, 212XLT, Trace Elliot preamp, and QSC power amp. The result was an AWESOME tone for SOME of the cover songs I was doing, but I was unable to tweak the tone any further. I had to max out knobs to hear the sound difference. In the end, both cabs were fighting each other and it was a pain in the butt live. In addition, the 410 always handled more power and I had to listen closely to how the 212 was doing. I could distort the 212 easily at every show. I ended up selling the 212XLT and bought an identical 410XLT. INSTANTLY better tone, easier to adjust, and I became much louder!

In the end, we are all trying to save you money.